Optical communication system using optical frequency code, optical transmission device and optical reception device thereof, and reflection type optical communication device

ABSTRACT

The invention dispenses with calibration of the optical frequency of the light source and permits the use of many codes without increasing the transmission bandwidth used. Let the optical frequency width of the light source be represented by FSR and the code length of every code be represented by FSR, the codes are made to be orthogonal to each other. The optical intensity-frequency characteristic of an n-th optical code signal is set to Cn(f)=(1+cos(2πsf/FSR+rπ/2))/2 (where s is an integer in the range from 1 to maximum number of codes/2, and r=0 or 1) to provide orthogonality between the optical code signals. Alternatively, optical frequency chips are sequentially assigned to chip sequences forming the optical code signals, the optical frequency of each chip “1” is output, and a filter is provided with an optical filtering characteristic of a concatenated code which is a repeated continuation of, for example a second-order Hadamard code word (0101) or (0011), and light emitted from the light source is passed through the filter to form the optical code signal. An encoding optical frequency region  31  and a decoding optical frequency region  32  are so chosen as to cover a range of drift of the source frequency. In FIG.  13 , ΔF 1  and ΔF 2  indicate drifts of the source frequency.

TECHNICAL FIELD

Of the present invention relates to an optical communications systemthat utilizes the OCDM (Optical Code Division Multiplex), QPSK(Quadrature Phase Shift Keying), or QAM (Quadrature AmplitudeModulation) technique which multiplexes plural data sequences into asingle data sequence that can be demultiplexed by use of differentoptical codes; furthermore, the invention pertains to an opticaltransmitter, an optical receiver and reflective optical communicationequipment for use in the optical communications system.

BACKGROUND ART

With respect to a point-to-multipoint transmission on the PON (PassiveOptical Network) wherein two or more local offices are connected viaoptical fibers to a central office, there has been proposed a schemeaccording to which: each local office is assigned pseudo-randomspreading codes orthogonal to each other, and modulates an opticalsignal in accordance with the spreading codes assigned thereto andtransmits the modulated optical signal; and the central officemultiplexes such optical signals from the respective local offices andtransmits over a long distance. A description will be given below of aconventional technique for optical frequency coding in an opticalfrequency region by use of the spreading codes.

FIG. 1 schematically shows the configuration of one channel and opticalfrequency coding (wavelength coding) in the optical code divisionmultiplex communications system. At the transmitting side, a broadbandoptical signal 20 is emitted from a light source 10 for incidence to anencoder 11, which performs wavelength coding of the incident opticalsignal by permitting the passage therethrough of only wavelengthcomponents corresponding to selection wavelength components 31 of theencoder 11, providing an coded optical signal 21. The thus coded opticalsignal 21 is transmitted over an optical fiber 13 to a decoder 12 at thereceiving side. The decoder 12 similarly permits the passagetherethrough of only those codes of the optical signal 21 from thecorresponding encoder 11 which are selected according to selectionwavelength components 32 of the decoder, providing a decoded opticalsignal 22.

On the other hand, as shown in FIG. 1( c), when an optical signal isinput to the decoder 12 from an encoder which does not correspond to thedecoder and the input optical signal contains wavelength components 21′based on the selection wavelength components 31′ of the encoder, allchips (optical frequencies or wavelengths) of the input optical signalare not allowed to pass through the decoder 12 according to itsselection wavelength components 32; even if allowed to pass, some chipsof the input optical signal are allowed, and hence the optical signal isnot decoded into an appropriate optical signal but instead becomes anoptical noise 22′. The encoder 11 and the decoder 12 mentioned hereinare disclosed in non-patent document 1, for instance.

The wavelengths for use by the conventional encoder and decoder arespecific for them, and the wavelength of the input optical signal 20 tothe encoder 11 and the selection wavelength 31 of the encoder 11 are notallowed, in almost all cases, to deviate from their predeterminedabsolute wavelengths. This raises a problem that the receiving side isrequired to notify the transmitting side of the wavelength of theoptical signal to be sent and the selection wavelength 31 of the encoder11, whereas the transmitting side is required to calibrate thewavelength 20 of the light source 10 and the selection wavelength 31 ofthe encoder 11 in response to the notification.

A solution to this problem is proposed, for example, in non-patentdocument 2 and patent document 1 (issued Feb. 2, 1999). With theproposed method, light emitted from a broadband light source which has awavelength width of several dozen nanometers, such as LED (LightEmitting Diode), is input to a Mach-Zehnder or Fabry-Perot filter madeof a material with less temperature dependence of its selectionwavelength, wherein the input light is subjected to wavelength coding bythe selection of its wavelength through use of sine functions; that is,data sequences are each assigned a wavelength with a different period.

In conventional optical communications, a binary data sequence istransmitted using an intensity modulation scheme that represents thepresence or absence of an optical signal, depending on whether the valueof each piece of data sequence is a space or mark.

A proposal has also been made to apply a four-phase modulation techniquenow in use in radio communications to optical communications as well.This technique is to convert the optical phase of an optical signal ofone wavelength into one of four predetermined phases in accordance withtwo data sequences.

For optical transmission of two or more data sequences in multiplexedform there are available an “Optical FDM (Optical Frequency DivisionMultiplex) or WDM (Wavelength Division Multiplex) method. In WDM-PONemploying the optical wavelength division multiplex method, it isnecessary to adjust the wavelengths of optical signals to be sent fromrespective local offices to ensure accurate signal multiplexing anddemultiplexing. To avoid such wavelength adjustment, there has beenproposed an optical communications system in which each local officemodulates, according to data sequence, an optical signal received fromthe central office and sends back thereto the modulated optical signal(see, for instance, non-patent documents 3 and 4).

[Patent Document 1] Japanese Patent Application Kokai Publication No.H11-32029

[Non-Patent Document 1] Saeko Oshiba et al., “Experimental Study on BitRate Enhancement Using Time-Spread/Wavelength-Hop Optical Code DivisionMultiplexing,” 2002 Annual General Conference of the Institute ofElectronics, Information and Communication Engineers of Japan, B-10-80

[Non-Patent Document 2] T. Pfeiffer et al., “High Speed Optical networkfor Asynchronous Multiuser Access Applying Periodic Spectral Coding ofBroad Band Sources,” vol. 33, No. 25, pp. 2141-2142, 1997, ElectronicsLetters

[Non-Patent Document 3] Takeshi Imai et al., “The Inter-Operability ofWDN-PON System ONU Using a Reflective SOA,” 2002 Society Conference ofthe Communications Society of the Institute of Electronics, Informationand Communication Engineers of Japan, B-10-50

[Non-Patent Document 4] Satoshi Narukawa et al., “TransmissionCharacteristics of Wavelength Channel Data Rewriter Using SemiconductorOptical Amplifier,” 2003 Society Conference of Communication Society ofthe Institute of Electronics, Information and Communication Engineers ofJapan, B-10-51

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

The prior art disclosed in documents 2 and 3 refers to OCDM (OpticalCode Division Multiplexing) in which data sequences are each assigned awavelength selected with a different period equivalent to a code, butsince these optical codes corresponding to each data sequence (channel)are not mutually orthogonal to each other, the code assignment in anarrow optical frequency width containing a small number of periodscauses interference between the optical signals, resulting in a drop intheir S/N (Signal/Noise). For example, letting the optical frequencydifference to be assigned to the first data sequence and a referenceoptical frequency wavelength be represented by λ1 and λ0, respectively,a code is assigned over a wide optical frequency width including notonly one period of optical frequencies λ0 to λ0+λ1 but also multipleperiods of optical frequencies λ0 to λ0+2λ1, λ0 to λ0+3λ1 . . . ; a codeis assigned to the second data sequence over a wide optical frequencywidth including not only one period of optical frequencies λ0 to λ0+λ2but also multiple periods of optical frequencies λ0 to λ0+2λ2, λ0 toλ0+3λ2 . . . ; and a code is assigned to the third data sequence over awide optical frequency width including not only one period of opticalfrequencies λ0 to λ0+λ3 but also multiple periods of optical frequenciesλ0 to λ0+2λ3, λ0 to λ0+3λ3 . . . ; and codes are similarly assignedthereafter. In this way, the prior art improves S/N.

With the above method, however, when the number of wavelengths to beselected with the period of a sine function is small, inter-channelinterference between optical signals is not negligible, so that it isdifficult to multiplex channel optical signals corresponding to manydata sequences without degradation of the bit error rate. To suppressthe inter-channel interference between optical signals, the wavelengthwidth of the light to be emitted from the light source needs to be wideso as to multiplex wavelengths of a number sufficiently large toapproximate the wavelength width to infinity. The use of a broadbandlight source gives rise to the problems of waveform degradation andlimitation on the transmission bandwidth due to the influence ofwavelength dispersion on the transmission line, leading to theimpossibility of high-speed transmission. Since light of a widefrequency width is required, the wavelength dispersion degrades thesignal-to-noise ratio in the case of long-distance transmission.Further, separation of channels only by the sine-function period makesit impossible to increase the number of channels in the condition thatthe frequency width of the light to be emitted from the light source andthe optical frequency selectable by a filter are limited.

In the case of controlling the optical phase of an optical frequencysignal according to a modulation signal (data) by the application to theoptical communication of the four-phase modulation technique actuallyused in conventional radio communications, it is difficult at present tocontrol the optical phase with accuracies on the order of tens ofnanometers which is a few tenth of micrometer for the optical wavelengthand hence is sufficiently accurate.

In the optical wavelength division multiplex PON disclosed in non-patentdocument 3, the optical signal that is used in the local office to senddata to the central office is sent in non-modulated continuous lightform from the central office to the local office. Since the transmissionof this non-modulated continuous light from the central to local officeis not utilized for information transmission, the informationtransmission efficiency is low accordingly. The equipment set forth innon-patent document 4 does not transmit such non-modulated continuouslight from the central office, and hence it is better in informationtransmission efficiency than the equipment of non-patent document 3, butthe central office sends a downstream optical signal of a low extinctionratio and the local office reuses the downstream optical signal of lowextinction ratio for an upstream optical signal for informationtransmission. Hence, the downstream optical signal of low extinctionratio impairs the communication quality.

Means for Solving the Problem

The present invention has first through third aspects, each of whichuses a function Ci(f) of an i-th code and its complementary function(1−Ci(f)), which satisfy the following conditions:

Function Ci(f) is a periodic function which satisfies Ci(f)=Ci(f+FSRi),and the function Ci(f) takes the value in the range of 0 to 1;

Optical frequency width FSR is an optical frequency width that is acommon multiple of a repetition period of the function of each code inthe range from a predetermined optical frequency Fst to a predeterminedoptical frequency Fla;

The complementary function of the function Ci(f) is a function (1−Ci(f))obtained by subtracting the function Ci(f) from 1, and the functionsCi(f) and (1−Ci(f)) bear the relationship∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df, where ∫df is a definite integral withrespect to f for an arbitrary interval FSR from Fst to Fla; and

Function Ci(f), a function Cj(f) of an arbitrary j-th code except ani-th one, and the complementary function (1−Cj(f)) of the function Cj(f)bear the relationship ∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df.

According to the first aspect of the present invention that is appliedto optical code communication:

the transmitting side generates and transmits, for each piece of data ofa binary data sequence, an optical code signal whose opticalintensity-frequency characteristic is at least one of the function Ci(f)and its complementary function (1−Ci(f)) both corresponding to the valueof each piece of data of the i-th binary data sequence, at least overthe enough wide period FSR that satisfies orthogonal relation betweenthe functions; and

the receiving side regenerates from received optical signal a firstintensity difference signal corresponding to the difference between afirst intensity signal, corresponding to the optical intensity of anoptical signal whose optical intensity-frequency characteristic is Ci(f)based on the function Ci(f), and a second intensity signal correspondingto the optical intensity of an optical signal whose opticalintensity-frequency characteristic is (1−Ci(f)) based on thecomplementary function (1−Ci(f)); and regenerate the data sequence fromthe first difference signal.

According to the second aspect of the present invention which performs,for example, pseudo-orthogonal phase modulation:

and let Δf represent the remainder of the division of an arbitraryoptical frequency width equal to or narrower than the optical frequencywidth FSR by the repetition period FSRi of the function Ci(f), let aphase 2π(Δf/FSRi) represent a phase difference from the function Ci(f),and let Ci′(f) (=Ci(f+Δf)) represent a function with an opticalfrequency (f+Δf) different by said remainder Δf from the opticalfrequency of the function Ci(f) of the i-th code, the function Ci′(f),the function Cj(f) and its complementary function (1−Cj(f)) bear therelationship:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df;

the transmitting side separates a binary data sequence into multipledata sequences, then generates an optical signal whose opticalintensity-frequency characteristic is at least one of the function andits complementary function both corresponding to the value of each pieceof data of each data sequence corresponding to each code and combinesand transmits such optical signals as an optical code signal;

the receiving side detects, based on the functions corresponding to theabove-said separated data sequences and their complementary functions,optical intensity differences between the optical signals having theiroptical intensity-frequency characteristics based on the above-mentionedfunctions, respectively, and regenerates the separated data sequences.

According to the third aspect of the present invention which is appliedto reflective optical communication:

an optical signal whose optical intensity-frequency characteristic isthe function Ci(f) or its complementary function (1−Cj(f)) is input atleast the optical frequency width FSR, the input optical signal is inputto an encoder whose filtering optical frequency characteristic is basedon the function Ci(f) and which outputs an optical signal, and the inputoptical signal is input as well to a complementary encoder whosefiltering optical characteristic is based on the complementary function(1−Ci(f)) and which outputs a complementary optical signal; and

optical signals and their complementary optical signals are selectivelycombined according to each piece of data of the input binary datasequence and transmitted as an optical code signal.

[Effect of the Invention]

According to the configuration of the first aspect of the presentinvention, the function Ci(f) is continuously repeated, and if it iswithin a frequency range from Fst to Fla, optical code signals of theoptical frequency width FSR at an arbitrary position need only to betransmitted; therefore, even if a drift occurs in the optical frequencyfor the light source and the encoder of the optical transmitter and thedecoder of the optical receiver, it is not necessary to notify thetransmitting side of the transmission optical frequency from thereceiving side and adjust the optical frequency at the transmitting sideaccordingly. Furthermore, since the optical code signals whose opticalintensity-frequency characteristics Fi(f) are orthogonal to each otherare used for a plurality of data sequences, it is possible to multiplexmany optical code signals, in which case the optical frequency width ofevery optical code signal needs only to be equal to FSR and there is noparticular need for increasing the optical frequency width.

According to the configuration of the second aspect of the presentinvention, since the function of the optical intensity-frequencycharacteristic is controlled for each piece of data of the separateddata sequences, the control accuracy for modulation may be appreciablylower than in the case of modulating the optical phase of an opticalfrequency signal and the control can easily be implemented.

According to the third aspect of the present invention, there is no needfor sending a non-modulated downstream optical signal that is sent backas an upstream optical signal, and the optical intensity-frequencycharacteristic functions of the downstream and upstream optical signalsare orthogonal to each other; hence, irrespective of whether thedownstream data is mark or space, it is possible to output the upstreamoptical signal of mark or space at the same optical intensity—this doesnot lower the extinction ratio of the downstream signal and henceprecludes the possibility of the transmission quality deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) shows an example of the system configuration of aconventional optical code division multiplex method, FIG. 1( b) and FIG.1( c) show examples of light-source optical wavelength components,selection wavelength components of an encoded amount, its passagewavelength components, selection wavelength components of a decoder andits passage wavelength components;

FIG. 2 illustrates an example of the system configuration of a multiplexcommunications system to which the first mode of working of the presentinvention is applied, FIGS. 2( a) and 2(b) showing its opticaltransmitter and optical receiver, respectively;

FIG. 3 illustrates another example of the multiplex communicationssystem configuration to which the first mode of working of the inventionis applied, FIGS. 3( a) and 3(b) showing its optical transmitter andoptical receiver, respectively;

FIG. 4 is a configuration illustrating an example of the communicationssystem to which the first mode of working of the invention is applied;

FIG. 5( a) shows an example of a source frequency drift, FIG. 5( b) anexample of an optical frequency region for encoding, and FIG. 5( c) anexample of an optical frequency region for decoding;

FIGS. 6( a), 6(b) and 6(c) are graphs showing examples of spreadingcodes in Embodiment 1, respectively;

FIG. 7 is a diagram illustrating an example of an encoder for use inEmbodiment 2;

FIG. 8 is a diagram illustrating an example of a decoder for use inEmbodiment 2;

FIG. 9 is a diagram illustrating an example of filters forming anencoder/decoder for use in Embodiment 2;

FIG. 10( a) is a diagram showing a first-order Hadamard matrix, FIG. 10(b) a diagram showing a second-order Hadamard matrix, and FIG. 10( c) adiagram showing a recurrence formula of the Hadamard matrix;

FIGS. 11( a) and 11(b) are graphs showing examples of encoding codes(concatenated codes) corresponding to the second-order Hadamard matrixfor use in Embodiment 3;

FIG. 12 is a diagram illustrating an example of a decoder for use inEmbodiment 3;

FIGS. 13( a), and 13(b) and 13(c) are diagrams showing, by way ofexample, source optical frequency component, optical frequency regionsfor encoding, encoded optical signals, optical frequency regions fordecoding, and decoded optical signals in the cases where no sourcefrequency drift occurs, and where the source frequency drift occurs,respectively, in Embodiment 3;

FIG. 14 is a diagram illustrating an example of a filter forming anencoder/decoder for use in Embodiment 2;

FIG. 15 is a diagram illustrating another example of the decoder for usein Embodiment 3;

FIG. 16 is a diagram illustrating another example of the filter formingan encoder/decoder for use in Embodiment 3;

FIG. 17 is a diagram illustrating still another example of the filterforming an encoder/decoder for use in Embodiment 3;

FIG. 18 is a diagram showing an example of a variable delay line for usein the filter forming the encoder/decoder in Embodiment 3;

FIG. 19 is a diagram showing another example of the variable delay linefor use in the filter forming the encoder/decoder in Embodiment 3;

FIGS. 20( a), and 20(b) and 20(c) are diagrams showing, by way ofexample, the relationships between source optical frequency component,optical frequency regions for encoding, encoded optical signals, opticalfrequency regions for decoding, and decoded optical signals in the caseswhere no optical frequency regions for encoding drift occur, and wherethe source frequency and optical frequency regions for encoding driftoccur, respectively, in Embodiment 3;

FIG. 21 is a diagram illustrating still another example of the filterforming the encoder/decoder for use in Embodiment 3;

FIG. 22 is a diagram illustrating another example of the encoder for usein Embodiment 3;

FIG. 23 is a diagram illustrating another example of the decoder for usein Embodiment 3;

FIG. 24 is a diagram illustrating another example of the encoder for usein Embodiment 3;

FIG. 25 is a diagram illustrating another example of the decoder for usein Embodiment 3;

FIG. 26 is a diagram illustrating still another example of the decoderfor use in Embodiment 3;

FIG. 27 is a diagram illustrating a still further example of the decoderfor use in Embodiment 3;

FIG. 28 is a diagram showing an example of an encoder/decodercombination according to Embodiment 2;

FIG. 29 is a system configuration illustrating an example of thecommunications system to which the first mode of working is applicable;

FIG. 30 is a system configuration illustrating an example of thecommunications system according to Embodiment 2-1 of the second mode ofworking of the present invention;

FIG. 31 shows, by way of example, the relationships between the phasecorresponding to two values of piece of data and pseudo-carrier of atrigonometric function, FIGS. 31( a), 31(b), 31(c) and 31(d) showingsuch relationships in the cases where the phase is 0, π/2, π and 3π/2,respectively;

FIG. 32-1 is a diagram showing, by way of example, the relationships oflight source output, modulated output, filtering characteristic,filtered output and detected intensity at the receiving side in the caseof a 0-phase modulated output in Embodiment 2-1;

FIG. 32-2 is a diagram showing, by way of example, the FIG. 32-1relationships in the case of a π/2-phase modulated output;

FIG. 32-3 is a diagram showing, by way of example, the FIG. 32-1relationships in the case of a π-phase modulated output;

FIG. 32-4 is a diagram showing, by way of example, the FIG. 32-1relationships in the case of a 3π/2-phase modulated output;

FIG. 33 is a diagram illustrating an example of a phase modulation part130 in FIG. 30;

FIG. 34 is a diagram illustrating an example of an optical transmitterin Embodiment 2-2;

FIG. 35( a) is a diagram showing an example of an optical transmitter inEmbodiment 2-3, and FIG. 35( b) a diagram showing an example of amodified form of a modulator 132 in FIG. 35( a);

FIG. 36( a) is a diagram showing signal points on the coordinates inQPSK, and FIG. 36( b) is a table showing the relationships of a dataset, a coordinate point and a filter-selecting phase;

FIG. 37-1 is a diagram illustrating an example of an optical transmitterin the communications system according to Embodiment 2-4;

FIG. 37-2 is a diagram showing an example of an optical receiver for usein Embodiment 2-4;

FIG. 38( a) is a diagram showing signal points on the coordinates inQPSK, and FIG. 38( b) is a table showing the relationships of a dataset, the filter-selecting phase and intensity to outputs fromcomparators 241 and 242;

FIG. 39 is a diagram illustrating another example of the opticaltransmitter for use in Embodiment 2-4;

FIG. 40 is a system configuration illustrating an example of acommunications system according to Embodiment 2-5;

FIG. 41 shows examples of filtering characteristics in Embodiment 2-5,FIGS. 41( a), 41(b), 41(c) and 41(d) showing the characteristics in thecases of where the phase is 0, π/2, π and 3π/2, respectively;

FIG. 42-1 is a diagram showing, by way of example, the relationships ofthe optical source output, a modulated output, a filteringcharacteristic, a filtered output at the receiving side and detectedintensity to a 0-phase modulated output in Embodiment 2-5;

FIG. 42-2 is a diagram showing, by way of example, the FIG. 42-1relationships in the case of the π/2-phase modulated output;

FIG. 42-3 is a diagram showing, by way of example, the FIG. 42-1relationships in the case of the π-phase modulated output;

FIG. 42-4 is a diagram showing, by way of example, the FIG. 42-1relationships in the case of the 3π/2-phase modulated output;

FIG. 43( a) shows an example of an optical frequency characteristicfunction in the case where the chip number L=24, P=4, n=1 and S=6 inEmbodiment 2-5, and FIG. 43( b) shows an example of the function in thecase where S=3 in the FIG. 43( a);

FIG. 44 shows, by way of example, the relationships of the chip numberL, the phase shift amount P, a divisor S and Q and n in Embodiment 2-5,FIGS. 44( a), 44(b) and 44(c) showing the relationships in the caseswhere P=0, P=1 and P=2, respectively;

FIG. 45 is a diagram illustrating an example of an optical transmitteraccording to Embodiment 2-8;

FIG. 46 is a diagram showing, by way of example, optical chips of eachof S-chip light source in FIG. 45;

FIG. 47 is a system configuration illustrating an example of acommunications system according to Embodiment 2-9;

FIG. 48-1 is a diagram showing an example of an optical transmitter in acommunications system according to Embodiment 2-11;

FIG. 48-2 is a diagram showing an example of an optical receiver inEmbodiment 2-11;

FIG. 49 is diagram illustrating the functional configuration of anembodiment of reflective optical communication equipment according tothe third mode of working of the present invention;

FIGS. 50( a) and 50(b) are diagrams showing examples in which chipfunctions are used as the optical frequency characteristics in the thirdmode of working of the invention, respectively;

FIG. 51 is a diagram showing an example in which encoders 440M and 440Sin FIG. 49 have chip functions;

FIG. 52 is a diagram showing a functional configuration of anotherembodiment of the reflective optical communication equipment accordingto the third mode of working of the invention;

FIG. 53 is a diagram illustrating the functional configuration of anexample of this invention equipment in which transmitter and receivercircuits are provided in parallel;

FIG. 54 is a diagram showing an example of the chip function in thethird mode of working of the invention;

FIG. 55 is a diagram illustrating the functional configuration of anexample of optical communication equipment which is the communicatingpartner of the reflective optical communication equipment according tothird mode of working of the invention;

FIG. 56 is a diagram illustrating the functional configuration of anexample in which transmitter and receiver circuits each having atrigonometric function filtering characteristics are both provided;

FIG. 57 is a diagram showing the functional configuration of anotherexample of an optical combiner in FIG. 53;

FIG. 58 is a diagram illustrating the functional configuration ofanother example in which the transmitter and receiver circuits are bothprovided in the third mode of working of the invention;

FIG. 59 is a diagram illustrating the functional configuration of anexample in which the receiver circuit is connected in cascade to theoutput of the transmitter circuit in Embodiment 3-5;

FIG. 60 is a diagram illustrating the function configuration of anotherexample in which the receiver circuit is connected in cascade to theoutput of the transmitter circuit in Embodiment 3-5;

FIG. 61 is a diagram illustrating the function configuration of anexample in which the transmitter circuit is connected in cascade to theoutput of the receiver circuit in Embodiment 3-5; and

FIG. 62 is a diagram illustrating the function configuration of anotherexample in which the transmitter circuit is connected in cascade to theoutput of the receiver circuit in Embodiment 3-5.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be given, with reference to the accompanyingdrawings, of embodiments of the present invention, and in the followingdescription, corresponding parts are identified by like referencenumerals throughout the drawings and no description will be repeated.

First Mode of Working (Optical Code Division Multiplex)

The first mode of working of the present invention permitsimplementation of optical code division multiplex, but is applicable aswell to a single-data-sequence optical communication that does notinvolve optical code division multiplexing; however, the title of thissection includes the “Optical Code Division Multiplex” in parenthesesfor ease in distinguishing between this and other modes of working ofthe invention.

A description will be given first of examples of transmitting- andreceiving-side apparatuses to which the present invention is applicable.FIG. 2( a) shows an example of the light transmitting-side apparatus towhich the first mode of working is applied. Sets of light sources 10_(n) and encoders 11 _(n) are each connected via an optical fiber 14_(n) to a coupler 15, where n=1, 2, . . . , N(N being an integer equalto or greater than 2). A data sequence D_(n) is input to each encoder 11_(n), wherein it is encoded into an optical code signal, and the opticalcode signal is input via the optical fiber 14 _(n) to the coupler 15,wherein it is combined with other optical code signals, and from which acombined optical code signal is output. Shown in FIG. 2( a) is anexample in which the coupler 15 is disposed away from the encoders 11 ₁,. . . , 11 _(N) at different distances.

FIG. 3( a) shows an example in which the encoders 11 ₁, . . . , 11 _(N)and the coupler 15 are disposed at the same place and the light source10 is provided in common to the encoders 11 ₁, . . . , 11 _(N). Thearrangements of FIGS. 2( a) and 3(b) may be used in combination.

In the light receiving-side apparatus, as depicted in FIG. 2( b), themultiplexed optical code signal input to a splitter 16 is split into Noptical signals, which are input via optical fibers 18 ₁, . . . , 18_(N) to decoders 12 ₁, . . . , 12 _(N), respectively, by which theoriginal data sequences D₁, . . . , D_(N) are demultiplexed and decoded.The splitter 16 may be placed away from the decoders 12 ₁, . . . , 12_(N) at different distances. The splitter 16 and the decoders 12 ₁, . .. , 12 _(N) may be disposed at the same place as shown in FIG. 3( b); itis also possible to employ a combination of the arrangements shown inFIGS. 2( b) and 3(b).

Embodiment 1-1

FIG. 4 illustrates a single-channel communications system to whichEmbodiment 1-1 of the first mode of working of is applicable. Embodiment1-1 comprises, as is the case with the conventional opticalcommunications system, a light source 10, an encoder 11, a decoder 12,and an optical transmission line (optical fiber) 13; furthermore,Embodiment 1-1 is provided with a dispersion compensator 17 thatcompensates for frequency-dependent propagation delay time differencesdue to frequency dispersion of the optical transmission line by levelingoff delay times of respective frequency components of the optical codesignal between its transmission and reception. The optical frequencybandwidth over which the dispersion compensator 17 implementscompensation is broader than at least the optical frequency bandwidth ofthe optical code signal.

The light source 10 outputs an optical signal of an optical frequencywidth FSR corresponding to the code length FCL (a common multiple ofFSRi described later on) at least in the optical frequency region (fromoptical frequencies Fst to Fla) for encoding by the encoder 11.

An optical signal 20 emitted from the light source 10 is encoded by theencoder 11 into an optical code signal in the optical frequency region.Unlike an encoder used in the conventional optical communications systemthe encoder 11 for use in Embodiment 1-1 generates in the opticalfrequency region an optical code signal of the code length FCL which isequivalent to that of all encoding codes (code words) used in theoptical communications system. The optical code signal in theabove-mentioned optical frequency region has such properties asmentioned below. The intensity of an n-th optical code signal is afunction Cn(f) of an optical frequency f (hereinafter referred to alsoas an encoding code); the function Cn(f) takes a value from 0 to 1; theintegration value of the function Cn(f) for an interval of an arbitrarycode length FCL in the optical frequency region from Fst to Fla forencoding by the encoder 11 is a value obtained by dividing FCL by 2; andthe optical frequency characteristic of the light transmittance throughthe encoder 11 _(n) is, in general, a repetition of the same functionCn(f) at intervals of the code length FCL in the optical frequencyregion from Fst to Fla for encoding by the encoder. And the followingequations hold.Cn(f)=Cn(f+FCL) n=1, . . . , N  (1)∫Cn(f)=FCL/2  (2)

In the following description an optical code signal whose opticalfrequency characteristic function of optical intensity is Cn(f) willalso be denoted by Cn(f); that is, Cn(f) represents an n-th encodingcode, or n-th optical code signal. The term “n-th (optical code signal)”corresponds to the term “n-th (optical code signal) in other modes ofworking of the invention, and (function or encoding code) Cn(f)”corresponds to “(optical frequency characteristic function or code)Cn(f)” in other modes of working of the invention.

The decoder 12 for decoding the optical code signal generated by theencoder 11 in Embodiment 1-1 is such that for the n-th optical codesignal Cn(f) the decoder 12 _(n) continuously repeats generation of afunction (hereinafter referred to also as a decoding code) Dn(f) whoseone period is equal to the code length FCL in the optical frequencyregion for decoding: Dn(f) is expressed by the following equation.Dn(f)=Cn(f)−Cn′(f)  (3)where Cn′(f) is a complementary value of the optical intensity value ofthe n-th encoded code Cn(f), and a value of the function Cn′(f) is thecomplementary value of the function Cn(f); they bear the followingrelationship.Cn(f)+Cn′(f)=1  (4)

The scalar product of the value Cn(f) of the n-th optical code signal atthe optical frequency f and the decoding code Dn(f) from the decoderdecoding the n-th optical code signal is integrated with respect to theoptical frequency f for a continuous optical frequency regioncorresponding to the code length FCL within each of the opticalfrequency region for encoding by the encoder and the optical frequencyregion for decoding by the decoder; the resulting value is a non-zerofinite value FCL/4, which satisfies the following equation.∫Cn(f)·Dn(f)df=FCL/4  (5)

Incidentally, the integration of Eq. (5) is conducted over the opticalfrequency width FSR of the light source; in this example, FSR is just anatural-number multiple of the period FCL.

The scalar product of the n-th optical code signal Cn(f) and thedecoding code Dm(f) from the decoder 12 _(n) having decoded an m-thoptical code signal Cm(f) other than the n-th optical code signal Cn(f),(where m=1, . . . , N and except for m=n), is integrated over thecontinuous frequency region corresponding to the code length FCL withineach of the optical frequency region for encoding by the encoder and theoptical frequency region for decoding by the decoder; the resultingvalue is zero, which satisfies the following equation.∫Cn(f)·Dm(f)df=0 m≠n, m=1, . . . , N  (6)

As shown in FIGS. 2 and 3, assume that the number of data sequences is aplural number N, that the first, . . . , N-th data sequences areassigned first, . . . , N-th encoding codes, respectively, and that thefirst, . . . , N-th encoding codes are equal in their code length FCL.The optical frequency region for encoding by the encoder 11 _(n) is setto be broader than the code length FCL of the optical code signal; theencoder 11 _(n) generates and outputs the optical code signal Cn(f) inaccordance with the data of the n-th data sequence by encoding, in theoptical frequency region, the optical signal input from the light sourceand having the optical frequency width which is at least the code lengthFCL. The length of the optical code signal Cn(f) to be output is set atone code length FCL for each piece of data. For example, when the datais “1” (mark), the optical code signal Cn(f) is output with one codelength, and when the data is “0” (space), the optical code signal Cn(f)is not output. Incidentally, the mark and the space correspond to theone and the other of two kinds of modulation unit signals.

As will be seen from Eqs. (3) and (5), the decoder 12 _(n), whichdecodes the n-th data sequence from the optical signal havingmultiplexed thereinto optical code signals of the N data sequences,integrates the scalar products of the input optical code multiplexedsignal and the n-th code signal Cn(f) and its complementary optical codesignal Cn′(f), then detects the difference between the integratedvalues, and outputs “1” or “0” as decoded data, depending on whether thedifference is equal to or greater than a predetermined value or smallerthan the predetermined value.

As described above, in Embodiment 1-1, unlike in the prior art examplewhich uses a different wavelength period for each data sequence, thecode length FCL that is equal to the optical frequency width over whichall optical code signals are orthogonal to each other is identical, andthe optical frequency characteristic of the transmittance of the encoder11 _(n) is such that Cn(f) continuously repeats in the optical frequencyregion FSR for encoding by the encoder, and the optical frequencycharacteristic of the transmittance of the decoder 12 _(n) is also suchthat Dn(f) continuously repeats in the optical frequency region Fst toFla for decoding by the decoder; therefore, each optical code signalkeeps the properties shown by Eqs. (1) and (2), and even if the intervalof integration is changed, the integrated value of the scalar product ofeach optical code signal in the decoder remains unchanged. Accordingly,in Embodiment 1-1 if the optical frequency width of the light source tobe encoded is constant and the optical frequency width is included ineach of the optical frequency region for encoding by the encoder and theoptical frequency region for decoding by the decoder, the optical codesignal emitted from the encoder corresponding to the light source havingits optical frequency changed is received by the decoder as an opticalsignal of the same intensity as that of the emitted optical signal, andno increase will be caused in the interference between the other opticalcode signals which do not correspond to this decoder. For example, asshown in FIG. 5( a), the optical frequency width of the output opticalsignal from the light source 10 is f_(L1) to f_(L2)=FSR, and thisoptical frequency width is a natural-number multiple of the optical codelength FCL over which almost codes are repeatedly generated (1 beingchosen as the natural number in this embodiment); this optical frequencywidth f_(L1) to f_(L2) is included in each of the optical frequencyregion for encoding by the encoder 11 _(n) and the optical frequencyregion for decoding by the decoder 12 _(n) as shown in FIGS. 5( b) and(c). Accordingly, even if the optical frequency of the output light fromthe light source 10 drifts as indicated, for example, by the brokenlines, as long as it stays within the optical frequency region forencoding and the optical frequency region for decoding, the optical codesignal of the drifted optical frequency is decoded by integrating theinput multiplexed optical code signal and the decoding code Dn(f) overFSR (equal to the code length FCL in this example) corresponding to theoptical frequency width of the light source; this integration and therelationships given by Eqs. (1) and (2) ensure generation of the samedecoded signal as is obtainable without the optical frequency drift, andsuppression of an increase in interference. Similarly, even if theoptical frequency region for encoding and the optical frequency regionfor decoding drifts, decoding can be achieved with high accuracy. Theoptical transmission bandwidth of the optical fiber for transmitting thecombined optical code signal from the combiner 15 (see FIGS. 2 and 3)needs only to be wider than the optical frequency width FSR of the lightsource to such an extent as to fully accommodate optical frequencyfluctuations of the light source. The optical frequency region forencoding and the optical frequency region for decoding may also be thesame as the above-mentioned optical transmission bandwidth. In otherwords, since codes are orthogonal to each other in the first mode ofworking of the invention, the optical frequency width FSR of the lightsource may be equal to the code length FCL of every code, in which casethe optical frequency width necessary for transmission over the opticalfiber may be an optical frequency width that is the sum of the codelength FCL and an optical frequency fluctuation of the light source.

The prior art disclosed in document 2 uses periodic codes of differentcode lengths of spreading codes, and requires, for canceling inter-codeinterference, an optical-band light source that is used to obtainoptical signal with a sine function over a sufficiently large number ofperiods. In Embodiment 1-1, however, such a broadband light source isnot required, and the emitted light from the light source 10 needs onlyto have an optical frequency width (period width) corresponding to thecode length FCL—this permits reduction of the transmission frequency(wavelength) width accordingly, solving the problems of waveformdeterioration by the influence of wavelength dispersion on the opticaltransmission line and limitations on the transmission bandwidth.

Furthermore, the provision of the dispersion compensators 17 alsoextenuates or minimizes the collapse of orthogonality between codes dueto differences in their transmission distance.

As described above, in Embodiment 1-1 the optical frequency region forencoding by the encoder has an optical frequency width greater than thecode length FCL of the optical code signal, the optical code has thecharacteristics expressed by Eqs. (1) to (6), and the dispersioncompensator 17 is disposed immediately before each decoder or behindeach encoder as indicated by the broken line in FIG. 2( b) or 2(a);accordingly, even if the optical frequency of the output signal from thelight source fluctuates within the optical frequency range for encodingby the encoder irrespective of the distance between the encoder and thedecoder and the optical frequency width of the output optical signalfrom the light source does not change, the output optical code signalfrom the encoder corresponding to the decoder is received by the latteras an optical code signal of the unchanged optical intensity and noincrease is caused in the interference by other optical code signalsthat do not correspond to this decoder—this allows the optical frequencyof the output signal from the light source to deviate from apredetermined absolute frequency, making it possible to avoid thenecessity for calibration of the optical frequency of the output signalfrom the light source.

Embodiment 1-2

Embodiment 1-2 of the first mode of working of the invention is aspecific operative example of Embodiment 1-1 and uses a trigonometricfunction as the encoding function C(f). In Embodiment 1-2, in the caseof using the smallest possible and invariable value a (which is apositive integer) to generate r′ codes, if the value a is taken as aninteger value in the range of 1 to a value N/r′ obtained by dividing themaximum number N of codes (the maximum number of local-office encoders)by r′ and if r is taken as 0, 1, . . . , r′−1 that is the remainder ofr′, the n-th optical code Cn(f) is used which is expressed by thefollowing equation.Cn(f)=(1+cos(2·π·a·f/FCL+r·π/2))/2  (7)

This optical code signal function Cn(f) takes a value from 0 to 1, andthe value of integration of the code signal function for an interval ofan arbitrary code length FCL in the optical frequency region forencoding by the encoder 11 _(n) is FSR/2; the optical frequencycharacteristic of the transmittance of the encoder 11 _(n) is arepetition of the function Cn(f) with a cycle of the code length FCL inthe optical frequency region for encoding by the encoder and satisfiesthe relations of equations (1) and (2).

A description will be given below, by way of example, of the case wherer′−2, that is, the remainder of division of r by r′ is 0 or 1 and atakes a value in the range of 1, . . . , N/2.

In FIG. 6 there are shown examples of the optical code signal Cn(f) inEmbodiment 1-2. The abscissa represents the optical frequency normalizedwith the code length FCL and the ordinate represents optical intensity;FIGS. 6( a), 6(b) and 6(c) correspond to values a=1, 2 and 3,respectively, and the broken and solid lines indicate optical codesignals corresponding to r=0 and r=1, respectively. The optical codesignal is one that single frequency optical signals corresponding torespective chips, except for Cn(f)=0, vary in intensity in analogfashion in order of their arrangement unlike the spreading codeconsisting of single-frequency optical signals of which values are each“1” or “0” corresponding to one of chips as shown in the FIG. 1prior-art example.

Used as the decoding code Dn(f) of the decoder 12 _(n) for decoding then-th optical code signal Cn(f) is expressed by the following equation.Dn(f)=(1+cos(2·π·a·f/FCL+r·π/2))−1  (8)

The scalar product of the n-th optical code signal Cn(f) and the n-thdecoding code Dn(f) for decoding the n-th optical code signal isintegrated over a continuous optical frequency region corresponding tothe code length FCL included in each of the optical frequency region forencoding by the encoder and the optical frequency region for decoding bythe decoder, the resulting value being a non-zero finite value FCL/4,and the scalar product of the n-th optical code signal Cn(f) and adecoding code Dm(f) of a decoder for decoding an m-th optical codesignal other than the n-th one over a continuous frequency regioncorresponding to the code length FCL included in each of the opticalfrequency region for encoding by the encoder and the optical frequencyregion for decoding by the decoder, the resulting value being zero;these values satisfy Eqs. (5) and (6) in Embodiment 1-1.

FIG. 7 shows an example of the configuration of the encoder 11 _(n) foruse in Embodiment 1-2. A Mach-Zehnder interferometer is used, as theencoder 11 _(n), which is made up of a pair of optical paths 41 and 42of different optical path lengths and a pair of couplers 43 and 44optically coupled thereto for coupling and splitting their opticalinputs into two which are fed to the optical paths. The light input tothe one input port of the coupler 43 is output via two output ports tothe optical paths 41 and 42. At the one output port of the coupler 43 ismainly provided light of an optical frequency component which is anintegral multiple of the optical frequency dependent on the optical pathdifference between the optical paths 41 and 42, whereas at the otheroutput port is provided mainly the other optical frequency component.This optical frequency selecting characteristic is a gentle one, notON-OFF-wise; therefore, for example, in FIG. 6( a) the selected(normalized) optical frequency is f1 and an optical output of a cosinefunction is provided whose intensity is 1 at the selected opticalfrequency f1.

Accordingly, the n-th optical code signal Cn(f) given by Eq. (7) isprovided as an output A from the one output port of the coupler 44. Fromthe other output port is provided an complementary optical code signalCn′(f) as an output B.

FIG. 8 shows an example of the configuration of the decoder 12 _(n) foruse in Embodiment 1-2. A Mach-Zehnder interferometer 55 is used which ismade up of a pair of optical paths 51 of different optical path lengthsand a pair of couplers 53 and 54 optically coupled thereto; a combinedoptical code signal is input to the Mach-Zehnder interferometer 55,which provides, as an output A at the one output port, the n-th opticalcode signal Cn(f) given by Eq. (7), and the optical intensity of theoutput is detected as an electrical signal by a detector 56 a. As theother output B from the Mach-Zehnder interferometer 55 is provided anoptical code signal Cn′(f) that is an complementary version of the n-thoptical code signal Cn(f) given by Eq. (7), and the optical intensity ofthe output Cn′(f) is detected by a detector 56 b as an electricalsignal. The output A corresponds to the scalar product of the inputcombined optical code signal and the encoding code Cn(f), whereas theoutput B corresponds to the scalar product of the input combined opticalsignal and an complementary code (1−Cn(f)) of the encoding code Cn(f)subtracted from 1. The output from the detector 56 a is obtained byintegrating the output A with respect to the optical frequency f of thelight-source optical frequency width included in the optical frequencyregion Fst to Fla for decoding, and the output from the detector 56 b isobtained by integrating the output B with respect to f over thelight-source optical frequency width FSR included in the opticalfrequency region Fst to Fla for decoding. An intensity differencedetector 57 subtracts the optical intensity detected by the detector 56b from the optical intensity detected by the detector 56 a, providingthe decoded output from the decoder 12 _(n). For example, data “1” or“0” is output, depending on whether or not the output from the intensitydifference detector 57 is equal to or greater than a threshold value.

As described above, since Embodiment 1-2 uses mutually orthogonaloptical code signals as in the case of Embodiment 1-1 unlike in theprior art example of document 2 using periodic codes of different codelengths for optical codes, the summation of scalar products of thedifferent optical code signals over their code lengths is zero—thisreduces inter-code interference as compared with the prior art exampleusing the non-orthogonal periodic codes.

In Embodiment 1-2, if the optical frequency width of the optical outputfrom the light source, which is encoded by the encoding code, isconstant and if the optical frequency of the output light from the lightsource is allocated within each of the optical frequency region forencoding by the encoder and the optical frequency region for decoding bythe decoder, there is no influence of optical frequency fluctuations ofthe light source as is the case with Embodiment 1-1. Unlike the priorart using non-orthogonal periodic codes, this embodiment does not callfor the use of a light source capable of emitting light over a periodlarge enough to ignore inter-code interference, that is, the opticalfrequency bandwidth of the output light from the light source need notbe wide, in particular, and the transmission bandwidth needs only to bewider than the optical frequency width FSR of the light source to suchan extent as to accommodate optical frequency fluctuations of the lightsource; therefore, this embodiment permits suppression of waveformdeterioration and limitations on the transmission bandwidth bothattributable to wavelength dispersion on the transmission line.

Furthermore, in Embodiment 1-2, a π/2 phase shift of the optical codesignal at the start position on the optical frequency axis, that is,changing r in Eq. (7) to 0 or 1, as well as changing the frequency f,that is, a in Eq. (7) makes it possible to increase the number ofencoding codes twice as large as in the case of changing only the period(a) for encoding.

[Modification of Embodiment 1-2]

Although in Embodiment 1-2 the optical code signal is output only whenthe data in the data sequence is “1” (mark), the optical code signal mayalso be output when the data is “0” (space). That is, the n-th opticalcode signal Cn(f) is output for the data “1” (mark) in the n-th datasequence and an complementary optical code signal Cn′(f) of the n-thoptical code signal Cn(f) is output for the data “0” (space). To performthis, the encoder 11 _(n) is provided with a switch 45 which is disposedsubsequent to the output-side coupler 44 as indicated by the brokenlines in FIG. 7; the switch 45 is supplied with the outputs A and B andis controlled by the value of each piece of data of the data sequence Dnto provide the output A for the data “1” (mark) and the output B for thedata “0” (space), generating a non-return-to-zero optical modulationsignal.

In this embodiment, Eqs. (9) and (10) hold for the mark.∫Cn(f)Dn(f)=FCL/4  (9)∫Cn(f)Dm(f)=0  (10)

For the space, Equations (11) and (12) holds.∫Cn′(f)Dn(f)=−FCL/4  (11)∫Cn′(f)Dm(f)=0 (12)

In this embodiment, too, integration is conducted over the opticalfrequency width FSR of the light source, but the width FSR is equal tothe code repetition optical frequency width FCL.

Accordingly, the optical intensity difference detector 57 outputs asignal composed of mark and space codes and hence twice (3 dB) largerthan in the above-described embodiment in which the intensity differencedetector 57 is supplied with only the “mark” optical signal and providesan output that goes to PCL/4 for the mark and to 0 for the space. Thisincreases the signal-to-noise ratio by 3 dB and hence permits reductionof the code length FCL accordingly, thereby lessening the influence ofwavelength dispersion of the transmission line. Incidentally, the switch45 may also be disposed at the stage preceding the input-side coupler43, as indicated by the broken line in FIG. 7, in which case the inputlight is input to either one of two input ports of the coupler 42,depending on the data Dn is mark or space, and the output light isderived from only one of the output ports of the output-side coupler 41.Also the signs of the mark (“1”) and the space (“0”) may be exchanged.In other words, the correspondence between the mark (“1”) and the space(“0”) and between the optical code signals Cn(f) and Cn′(f) may bearbitrary.

The encoder 11 _(n) may also be configured as depicted in FIG. 9. As isthe case with an LN modulator, there are formed on a planar lightwavecircuit substrate 46 formed of a material which has an electroopticeffect, such as LiNbO₃ crystal: two waveguides 47 and 48; couplers 43and 44 formed by bringing the waveguides close to each other atlocations adjacent their opposite end portions; and a pair of electrodes49 for applying an electric field to at least either one of two opticalpaths 41 and 42 formed by the waveguides between the couplers 43 and 44so as to provide a propagation delay difference between the two opticalpaths 42 and 41 by birefringence shift that is caused under the electricfield by the electrooptic effect. The voltage that is applied to theoptical path (waveguide) by the pair of electrodes 49 is adjusted suchthat the encoder 11 _(n) selects and outputs the optical frequency(wavelength) signal which satisfies Eq. (7) corresponding to eachoptical code signal Cn(f).

The decoder 12 _(n) can similarly be formed as a Mach-Zehnderinterferometer or filter by forming optical paths 51, 52 and thecouplers 53, 54 on the planar lightwave circuit substrate asparenthesized in FIG. 9. In this instance, the voltage to be applied tothe electrodes 49 is so adjusted as to satisfy Eq. (8).

With the FIG. 9 configuration, it is possible to change the encodingcode Cn(f) or decoding code Dn(f) by changing the voltage which isapplied to the electrodes 49—this eliminates the need for forming adifferent encoder-decoder pair for each encoding code, and hence permitsreduction of the device manufacturing costs.

Further, as depicted in FIG. 9, a pair of encoders 11 _(n) and 11 _(m)(n≠m) are integrated on the same planar lightwave circuit substrate 46whose temperature varies uniformly throughout it, and these encoders 11_(n) and 11 _(m) generate n- and m-th optical code signals Cn(f) Cm(f)which are common in the value a but different in the value r in Eq. (7),respectively. Since the two optical code signals Cn(f) and Cm(f) areidentical in their optical frequency characteristic and have a phasenumber difference of π/2, unsimultaneous occurrence of temperaturevariations in the encoders 11 _(n) and 11 _(m) for encoding the opticalcode signals, respectively, changes their refractive indexes and opticalpath lengths and hence causes their optical frequency for filtering todrift, resulting in deterioration of the value of cross-correlationbetween the optical code signals Cn(f) and Cm(f). With the FIG. 9configuration, however, it is possible to suppress the deterioration ofthe correlation between the optical code signals by temperaturevariations since the encoders 11 _(n) and 11 _(m) are mounted on thecommon planar lightwave circuit substrate which undergoes uniformtemperature variations.

Embodiment 1-3

In Embodiment 1-3 of the first mode of working of the invention, theintensity of the chip, which is each optical frequency component of theoptical code signal, goes to a 1 or 0. The configuration of thecommunications system to which Embodiment 1-3 is applicable may be thesame as the FIG. 4 configuration.

The optical code signals, i.e. first to N-th optical code signals,generated by the encoder 11 _(n) in Embodiment 1-3 have the same codelength FCL and are orthogonal to each other as in Embodiments 1-1 and1-2; and they have such properties as mentioned below. The numbers ofchips “1” and chips “−1” in a string of chips of the code length FCLarbitrarily taken out of a continuously repeated concatenation of theencoding code Cn(f) of the code length FCL are equal (the same number),and the numbers of chips which simultaneously go to “1s” and “−1s”,respectively, at the same positions in strings of chips of the codelength FCL arbitrarily taken out of different concatenations ofdifferent encoding codes are equal to each other. In the case of a codecomposed of such chips, the code length is a mere abstract number withno unit. Accordingly, it can be said in the above-described embodiments,too, that the code length is the optical frequency width FCL over whichall codes repeat.

Such a code can be generated by use of the Hadamard code. FIG. 10( a)shows a first-order Hadamard matrix H₁, FIG. 10( b) a second-orderHadamard matrix H₂, and FIG. 10( c) a recurrence formula H_(n) of theHadamard matrix. A Hadamard code word is a selected row from theHadamard matrix, except the first row, and its 0 and 1 are substitutedwith 1 and −1, respectively. In the second-order Hadamard matrix, theHadamard code is composed of a code 2 [0101] in the second row of thematrix, a code 3 [0011] in the third row, and a code 4 [0110] in thefourth row. Continuously repeated concatenations of these codes 2 to 4are [ . . . 0101010101 . . . ], [ . . . 001100110011 . . . ], and [ . .. 011001100110 . . . ], respectively. In this case, since theconcatenation of code 3 and the concatenation of code 4 have their1-chip codes shifted from each other, they constitute identical encodingcodes, and Embodiment 1-3 uses only one of them.

In the encoder 11 _(n), consecutive optical frequencies are sequentiallyassigned to respective chips of such a concatenated code in the order oftheir arrangement, and those optical frequency components of the inputlight corresponding to the chips “1” are selected, that is, encoded. Theselection optical frequency components of the encoder corresponding tothe concatenation of the code C₁=(0101) are such as depicted in FIG. 11(a), and the selection optical frequency components of the encodercorresponding to the concatenation of the code C₂=(0011) are such asdepicted in FIG. 11( b).

The encoder 11 _(n) is configured to receive from the light source alight input of an optical frequency width F_(w) equal to or a littlelarger than a natural-number multiple of the code length FCL and hencefilter the optical frequency signal (component) corresponding to eachchip of the concatenation of the encoding coder Cn(f) and output thefiltered optical frequency signals as the optical code signal Cn(f)corresponding to the N-th data sequence, or output the optical codesignal Cn(f) of the N-th data sequence in which the optical frequencysignal (component) corresponding to each chip is ON for the data “1” ofthe N-th data sequence and OFF for the data “0”. The thus encodedoptical code signals each possess the above-mentioned properties of thechip string arbitrarily taken out from the concatenation of codes,maintaining orthogonality between different optical code signals.

The decoder 12 _(n) also filters the optical frequency component(signal) of the input light corresponding to the concatenated code as isthe case with the encoder 11 _(n), and performs decoding over thefrequency width corresponding to at least the code length FCL. FIG. 12shows an example of the configuration of the decoder 12 _(n). Themultiplexed optical code signal is split by a splitter 61 into two forapplication to filters 62 a and 62 b; the filter 62 a filters theoptical frequency signals of the same order of output from thecorresponding encoder 11 _(n), that is, the optical frequency signalscorresponding to the same chips as in the output from the correspondingencoder, and the filter 62 b filters the optical frequency signalscorresponding to complementary versions of the encoded codes of thecorresponding encoders 11 _(n), that is, the optical frequency signalscorresponding to chips not selected by the encoder 11 _(n). The opticalintensity of the optical frequency signal filtered or selected by thefilter 62 a and the optical intensity of the optical frequency signalfiltered or selected by the filter 62 b are detected by detectors 63 aand 63 b, respectively, and the output from the detector 63 b issubtracted by an intensity difference detector 64 from the output fromthe detector 63 a. In this way, optical code signals corresponding toconsecutive chips forming at least the encoded code are decoded from theoutput light of the encoder 11 _(n).

Referring next to FIG. 13, a description will be given of how Embodiment1-3 excludes the influence of the source frequency drift. FIG. 13( a)shows the state in which there is no source frequency drift. The opticalsignal 20 of the source frequency width F_(W) corresponding to or alittle larger than the code length FCL is output from the light source,and those optical frequency components of the optical frequency signal20 corresponding to the chips “1” are filtered (encoded) by the encoder11 _(n) in its encoding optical frequency region 31, by which an opticalcode signal 21 is generated. The decoder 12 _(n) filters the inputoptical code signal 21 in its decoding optical frequency region 32 toprovide a decoded optical signal 22.

When the source frequency drifts by ΔF₁ as shown in FIG. 13( b), theoptical signal 20 input to the encoder 11 _(n) is shifted by ΔF₁ in thesame direction as that of the source frequency drift in the encodingfrequency region 31, in which the input optical signal is encoded intothe optical code signal 21, whereas in the decoder 12 _(n) the inputmultiplexed optical signal 21 is also shifted by ΔF₁ in the samedirection as that of the source frequency drift in the decodingfrequency region, in which the input signal is decoded into an outputoptical signal 22.

Similarly, even when the source frequency drift is ΔF₂ larger than inthe above as shown in FIG. 13( c), if the drifted optical signal 20remains within the encoding frequency region 31 and within the decodingfrequency region 32, the optical signal is shifted by ΔF₂ in bothregions and encoded and decoded, respectively; in either case, theoptical code signal 21 held orthogonal to a different optical codesignal as described previously.

Incidentally, since it is absolutely impossible for the optical signalto be negative in intensity, it can easily be understood that replacingthe chip value “−1” by “0”, the value of definite integrals of scalarproducts of the code Cn(f) and the decoding code Cn(f) and itscomplementary code (1−Cn(f)) with respect to the optical frequency fover the source frequency F_(W), respectively, bear the relationshipexpressed by the following Equation (13).∫Cn(f)·Cn(f)df>∫Cn(f)·(1−Cn(f))df  (13)[Examples of Filters of Encoder/Decoder in Embodiment 1-3]

A description will be given below of examples of the filter for use inthe encoder/decoder for continuously encoding/decoding optical signalsin Embodiment 1-3. FIG. 14 depicts an example of the filter. Multiplevariable couplers 71, whose coupling factors can be set arbitrarily, areconnected in cascade via delay lines 72, and optical outputs from thoseoutput ports of the couplers 71 to which the delay lines 72 are notconnected are provided via phase shifters 73 to a combining coupler 74,from which a filtered optical output signal is derived. The opticalfrequency for filtering by this filter may a continuous repetition of anarbitrary sequence of optical frequencies with a predetermined period(FCL) as disclosed, for example, in Sasayama et al., “Photonic FDMMultichannel Selector Using Coherent Optical Transversal Filter,”Journal of Lightwave Technology, Vol. 12, No. 4, 1994, pp. 664-669. Thatis, it is possible to use the encoding light filtering frequencycharacteristic function Cn(f) that optical frequencies corresponding torespective chips “1” of the encoding code are continuously repeated witha period of the code length FCL.

In Embodiment 1-3, since the optical code signal can be sent over theoptical frequency width corresponding to the encoding code length FCLwithout concatenation, the required source optical frequency width canbe made smaller than in the case of the conventional method using thesine function for encoding—this lessens the influence of wavelengthdispersion on the transmission line. Moreover, as is the case withEmbodiment 1-1, the provision of the dispersion compensator 17 alsoextenuates the collapse of orthogonality between optical code signalsdue to differences in their transmission distance.

As described above, in Embodiment 1-3, even if the source frequencyshifts, as long as it remains within the optical frequency range forencoding by the encoder, and the optical input has at least thefrequency width corresponding to the code length FCL of the encodingcode, the optical code signal from the encoder is received by thecorresponding decoder at the same input intensity as the sourcefrequency does not shift and is held orthogonal to optical inputs fromother encoders noncorresponding to the decoder to thereby cancel theinter-code interference. This permits implementation of an opticalcommunications system free from the necessity for calibrating the sourcefrequency.

[Modified Decoder in Embodiment 1-3]

FIG. 15 shows a modified form of the decoder 12 _(n) in Embodiment 1-3.A filter 62 provides the output A by filtering the optical frequencysignals corresponding to the optical frequency selective characteristicsof the encoder 11 _(n), that is, the optical frequency signalscorresponding to the chips “1”, and it provides the output B byfiltering the optical frequency signals corresponding to complementarycharacteristics of the optical frequency selective characteristics ofthe encoder 11 _(n), that is, the optical frequency signalscorresponding to the chips unfiltered by the encoder 11 _(n). Theoutputs A and B from the filter 62 are input to detectors 63 a and 63 b,respectively, for detecting their optical intensity, and the detectedare applied to an intensity difference detector 64, which subtracts theoutput of the detector 63 b from the output of the detector 63 a andprovides the subtracted output. The output from the intensity differencedetector 64 is branched for input as well to a controller 65, and inaccordance with the input thereto the controller 65 shifts the filteringoptical frequency of the filter 62 to maximize the output from theintensity.

FIG. 16 illustrates a specific operative example of the filter 62 inFIG. 15. The filter 62 can be used also as an encoder. The illustratedfilter 62 is formed by a multistage Mach-Zehnder interferometercomprised of: two optical paths 76 a and 76 b having their optical pathlengths made different by the insertion of delay lines 75 in one of thetwo optical paths; couplers 77 interconnected via the two optical paths76 a and 76 b, for combining and splitting light from the optical paths;and phase shifters 73 inserted in one of the two optical paths 76 a and76 b. The filter periodically selects optical frequencies. The opticalfrequency for filtering (filtering optical frequency characteristicfunction) by the filter 62 may be a continuous repetition of anarbitrary sequence of optical frequencies with the predetermined period(FCL) as disclosed, for example, in Jinguji et al., “Synthesis ofCoherent Two-Port Lattice Form Optical Delay-Line Circuit,” Journal ofLightwave Technology, Vol. 13, No. 1, 1995, pp. 73-82. The filter 62provides the output A and the output B from the one and the other of twooutput ports of the last-stage coupler 77, respectively. When the filteris used as the encoder, for example, only the output A is provided asthe filtered optical code signal from one of the two output ports.

In the filter of FIG. 16 both port outputs of the coupler of each stageare fed to the next stage and the output A from the last-stage coupler77 is one-half the input, whereas in the filter of FIG. 14 the outputdecreases by 1/number of couplers 71; accordingly, when used as anencoder, the filter shown in FIG. 16 is lower in the splitting loss bycouplers.

As described above, the filter 62 in FIG. 15 may be configured as shownin FIG. 16 or 14, and its filtering optical frequency can be shiftedsimply by adjusting one or more of the delay of the delay line, theshift amount of the phase shifter and the splitting ratio of thevariable coupler. The maximum value of adjustment in this case needsonly to be equal to the code length FCL since the optical frequencywidth required for orthogonality of codes is FCL in this example.

FIG. 17 shows an example of the filter 62 whose filtering opticalfrequency is adjustable. This example uses delay lines 75′ whose delayamounts can be varied by an electrode or heater for causing a refractiveindex change through utilization of a change in the birefringent indexby the application of an electric field as described previously withreference to FIG. 9 or through utilization of the thermooptic effect bytemperature—this permits adjustment of the optical path lengthdifference between the optical paths 76 a and 76 b. The filter is formedby a multistage Mach-Zehnder interferometer comprised of the opticalpaths 76 a and 76 b, two couplers 77 for coupling and splitting lightfrom the two optical paths, and the variable delay lines 75′ inserted inthe one optical path. This filter varies the delay amount of thevariable delay line 75′ to shift the optical frequency to be selected byeach Mach-Zehnder interferometer forming the filter, thereby changingthe encoding code of the encoder 11 _(n) and the decoding code of thedecoder and shifting the optical frequencies of the encoded and decodedcodes.

The delay line of the filter for use in the encoder 11 _(n) or decoder12 _(n), whose delay amount is variable and adjustable, that is, avariable delay line is such as shown in FIG. 18, in which multiple delaylines 75 ₁ to 75 _(p) of different delay amounts (the delay amount ofthe delay line 75 _(p) being zero) are connected in parallel between twoswitches or couplers 77 and 78 for selectively connecting multipleoptical paths to a single optical path. An optical signal is output viaone delay line 75 _(p) (where p=1, . . . , P) having selected theoptical input. The use of switches in this case excludes the possibilityof incurring an optical loss by the splitting loss of the couplers.Since a desired delay is provided by selectively switching multipledelay lines of different delay times, instead of utilizing thethermooptic effect or a birefringent index change by the application ofan electric field, it is possible to cause a larger change in the delay.

FIG. 19 shows another example of such a delay line configuration, whichis formed by a multistage connection of an optical path provided with adelay line 81, an optical path with no delay line and a switch 82 forswitching between these delay lines, and in which the amount of delay onthe optical input is changed by selectively switching the switches 82between the two kinds of delay lines.

The delay lines 81 may preferably be different in delay amount. Thedelay of the optical input is adjusted by selectively changing thecombination of delay lines 81 for the passage therethrough of theoptical input. Though smaller in the number of delay lines used, theillustrated delay structure produces the same effect as is obtainablewith the FIG. 18 example.

Referring next to FIG. 20, it will be described below that Embodiment1-3 permits excellent decoding irrespective of a drift in the opticalfrequency for encoding by the encoder 11 _(n). In FIG. 20 the partscorresponding to those in FIG. 13 are identified by the same referencenumerals as in the latter. FIG. 20( a) shows the state in which there isno drift in the optical frequency for encoding by the encoder (opticalfiltering frequency characteristics), and the encoder and decoderoperate in the same manner as in the case of FIG. 13( a). When theoptical frequency for encoding by the encoder drifts ΔF₁ as depicted inFIG. 20( b), the optical input is encoded by filtering into an opticalcode signal 21′. When the optical frequency for encoding undergoes arelatively large drift ΔF₁ as shown in FIG. 20( c), the optical input isencoded by filtering into an optical code signal 21″.

Either of the optical code signals 21′ and 21″ has the code length FCL;accordingly, the optical code signals 21′ and 21″ have the sameproperties as does the string of consecutive chips extracted from theconcatenated code over the source frequency FSR (denoted by F_(W) inFIG. 20) as described previously. Therefore, the decoder 12 _(n) selectsrespective optical frequency components of the optical code signals 21′and 22″, and outputs decoded signals 22′ and 22″ as shown in FIGS. 20(b) and (c), respectively; thus, this embodiment ensures satisfactorydecoding. When the optical frequency for decoding drifts, the decoder 12_(n) described above with reference to FIG. 15 is used to shift theoptical frequency for decoding, by which satisfactory decoding can beachieved.

As described above, according to Embodiment 1-3, even if one or both ofthe source frequency, the optical frequency region for encoding 31 andthe frequency region for decoding 32 drift, as long as the opticalfrequency width of the optical signal from the light source lies withinthe regions 31 and 32 (the region 32 including the shift-controlledregion), no degradation is caused in the optical intensity of the inputto the decoder and the orthogonality to other optical code signals isalso retained—this enables the decoder to perform satisfactory decoding.

[Modifications of Encoder and Decoder of Embodiment 1-3]

FIG. 21 illustrates another example of the filter for use in the encoder11 _(n) and decoder 12 _(n) of Embodiment 1-3. The optical input is fedto a filter 84; the filter 84 outputs optical frequency signals ofrespective chips forming the encoding code to different ports, andoutputs to the same port those optical frequency signals which arespaced apart the optical frequency corresponding to the code length. Forexample, assuming that the encoded code is composed of four chips andthat optical frequencies F₁, F₂, F₃ and F₄ are sequentially assigned tothe chips in the order of arrangement, an optical signal of an opticalfrequency F₁+qFCL (where q=0, 1, 2, . . . ) is output to a port 1, andoptical signals of optical frequencies F₂+qFCL, F₃+qFCL and F₄+qFCL areoutput to ports 2, 3 and 4, respectively. As the filter that repeatedlyoutputs optical signals of consecutive frequencies to different ports asmentioned above, an AWG (Arrayed Waveguide Graiting) can be used whichis of the type that the product of the number of optical signals to besplit and their frequency interval and the free space range of theoptical signals to be output to the same port are equal to the codelength FCL. Incidentally, while the Free Space Range defined for AWG isabbreviated as FSR, it is not the optical frequency width FSR usedherein but, by the definition of FSR in this specification, it isrepresented as C/FCL (C: light speed).

The ports, at which the optical frequency signals corresponding to theselection frequency components, that is, chips “1” of the encoding code,of the filter 84 are provided, are connected via optical paths 85 to acoupler or combiner 86 a, and the output from the coupler or combiner 86a is provided as the output A. The ports, at which the optical frequencysignals corresponding to the chips “−1” of the encoding code, that is,unselected optical frequency signals, are provided, are connected viaoptical paths 87 to a coupler or combiner 86 b, and the output from thecoupler or combiner 86 b is provided as the output B. The FIG. 21example shows the path connection for the encoding code C₂=(0011)depicted in FIG. 11( b). The ports 1 and 2 for F₁+qFCL and F₂+qFCL areconnected via the optical paths 87 to the combiner 86 b, whereas theports 3 and 4 for F₃+qFCL and F₄+qFCL are connected via the opticalpaths 85 to the combiner 86 a.

It will easily be understood that the filter of the FIG. 21configuration can be used as a filter for either of the encoder 11 _(n)and the decoder 12 _(n).

In the encoder 11 _(n) either one of the couplers or combiners 86 a and86 b and the optical paths 85 or 87 corresponding thereto can beomitted. Rather than the couplers or combiners 86 a and 86 b which splitor couple light irrespective of its optical frequency and hence causes asplitting loss, it is preferable to use arrayed waveguide grating ascombiners not as filters in the above example since the optical loss bythe splitting loss can be reduced. This filter is smaller in the numberof parts than the filter of FIG. 14, and hence has the advantage of lowoptical loss.

The encoder 11 _(n) may be configured as shown in FIG. 22. The samefilter as that 84 in FIG. 21 is used, and its ports are connected to acombiner 92 via switches 91 ₁, . . . , 91 _(E) (where E is the number ofchips forming the encoding code) which selectively permit or inhibit thepassage therethrough of light to optical paths 89 ₁, . . . , 89 _(E).Those of the switches 91 ₁, . . . , 91 _(E) which correspond to thechips “1” of the encoding code are turned ON and those corresponding tothe chips “−1” are turned OFF.

FIG. 23 shows an example of the decoder formed using the filter 84. Thedecoder is provided with switches 93 ₁, . . . , 93 _(E) through whichthe optical paths 89 ₁, . . . , 89 _(E) connected to respective ports ofthe filter 84 are selectively connected to either one of combiners 92 aand 92 b, and those of the switches 93 ₁, . . . , 93 _(E) whichcorresponds to chips “1” are connected the combiner 92 a, whereas thosecorresponding to the chips “−1” are connected to the combiner 92 b. Theoutputs from the combiners 92 a and 92 b are applied to detectors 63 aand 63 b, respectively; the subsequent stage is the same as shown inFIGS. 12 and 15.

The configurations of FIGS. 22 and 23 can be adapted to generate/decodean arbitrary optical coded/decoded signal with one encoder/decoder.

FIG. 24 illustrates an other example of the encoder using the filter 84.The optical input is fed via an optical circulator 94 to the filter 84;the optical paths 89 ₁, . . . , 89 _(E) connected at one end to therespective ports of the filter 84 are connected at the other end toselective reflectors 95 ₁, . . . , 95 _(E) capable of selectivelyreflecting light, and the light reflected by the reflectors 95 ₁, . . ., 95 _(E) back to and combined by the filter 84, from which the combinedlight is output via the optical circulator 94 separately of the opticalinput. Those of the selective reflectors 95 ₁, . . . , 95 _(E) whichcorrespond to the chips “1” of the encoding code of the encoder are setfor reflection, whereas those reflectors corresponding to the chips “−1”are set for no-reflection.

Another example of the decoder using the filter 84 is depicted in FIG.25, in which the parts corresponding to those in FIGS. 23 and 24 areidentified by the same reference numerals as those in the latter.Connected to the optical paths 89 ₁, . . . , 89 _(E) at one end thereofare pass/reflect switches 96 ₁, . . . , 96 _(E) that can be set totransmit therethrough light or reflect it off. The light allowed to passthrough the pass/reflect switches 96 ₁, . . . , 96 _(E) is combined bythe combiner 92 a and then fed to the detector 63 a. The light reflectedoff the pass/reflect switches 96 ₁, . . . , 96 _(E) is combined by thefilter 84, from which the combined output is provided via the circulator94 to the detector 63 b. Those of the pass/reflect switches 961, . . . ,96E which correspond to the chips “1” of the encoding code are set fortransmission, whereas those corresponding to the chips “−1” are set forreflection. With the illustrated configuration, it is possible todecrease the number of combiners used in the decoder of FIG. 23 by one.

It is desirable to insert attenuators 90 ₁, . . . , 90 _(E) each in oneof the optical paths 89 ₁, . . . , 89 _(E) as indicated by the brokenlines in FIGS. 21 to 25 to provide an optical loss corresponding to theoptical intensity ratio of the output from the filter 48 for eachoptical path, thereby leveling off an intensity difference of severaldBs for each optical path which is likely to be caused by the arrayedwaveguide grating AWG. This permits reduction of noise resulting from anintensity variation for each optical frequency.

FIG. 26 illustrates still another example of the decoder using thefilter 84. This example is shown as being applied to the decoding ofoptical code signals of the code words C₁=(0101) and C₂=(1100) shown inFIGS. 11( a) and (b). The intensity outputs of optical frequency signalscorresponding to the chips “1” of the optical code signal in thedetectors 63 ₁, . . . , 63 ₄ are applied as positive outputs tointensity difference detectors, and the intensity outputs of the opticalfrequency signals corresponding to the chips “−1” are applied asnegative outputs to the intensity difference detectors. The outputs fromdetectors 63 ₁ and 63 ₃ are applied to positive input terminals of theintensity difference detector 64 _(n) for the code word C₁, and theoutputs from the detectors 63 ₂ and 63 ₄ are applied to negative inputterminals of the difference detector 64 _(n), which conducts an additionand a subtraction using the inputs to the positive and negative inputterminals as additive and subtractive inputs, respectively, and providesthe result of addition and as a decoded signal output. The outputs fromdetectors 63 ₁ and 63 ₂ are applied to positive input terminals of theintensity difference detector 64 _(m) for the code word C₂, and theoutputs from the detectors 63 ₃ and 63 ₄ are applied to negative inputterminals of the difference detector 64 _(m), which adds and subtractsthem, providing a decoded signal output. Alternatively, the intensitydifference detector 64 may also be adapted to calculate the sum of theoutputs from the detectors corresponding to the chips “1” of the encodedcode and the sum of the detector outputs corresponding to the chips “−1”and subtract the latter from the former to provide the decoded signaloutput.

This configuration eliminates the need for splitting light for eachencoding code (code word) for decoding, and hence permits reduction ofoptical loss accordingly. As described previously with reference to FIG.15, it is preferable in this instance, too, to control the filteringoptical frequency of the filter 84 in such a manner as to maximize onedecoded signal output.

FIG. 27 illustrates another example of the decoder using the filter 84.This example uses electrical delay means as the dispersion compensator.The outputs from the detectors 63 ₁, . . . , 63 ₄ connected to theoutput paths of the filter 84 are applied to a switch 97, which outputscode words added with positive and negative signs, and the detectedoutputs corresponding to respective chips of the encoded code are eachelectrically delayed by delay means 98 to equalize different arrivaltimes of chips due to different delay times of respective opticalfrequencies during transmission, and the delay-equalized detectoroutputs are added by the intensity difference detectors 64 _(n) and 64_(m), respectively. This example implements a dispersion compensation ofelectrical signals, and hence lessens the burden of dispersioncompensation of optical signal. By changing the setting for theswitching operation of the switch 97, arbitrary encoded codes can bedecoded. The delay means 98 can be dispensed with.

[Other Modifications]

In the encoder 11 _(n) a filter of the type providing an output A and ancomplementary output B is used, and the switch 45 is connected to theoutput side of the filter as indicated by the broken line in FIG. 16,and the switch 45 is controlled by a data sequence D_(n) to provide theoutput A or B, depending upon whether the data is the mark (“1”) orspace (“−1”); thus, the output is provided as a non-return-to-zerosignal. In this instance, too, the switch 45 may be connected to theinput side of the filter as described previously in respect of FIG. 9.Further, as in the case of adding the broken-line part in FIG. 7, a gainof 3 dB can be obtained.

As depicted in FIG. 28, a pair of encoder 11 _(n) and decoder 12 _(m) isintegrated on a planar lightwave circuit substrate 46 common to them.The n-th encoding code of the encoder 11 _(n) and the m-th decoding codeof the decoder 12 _(m) are common in the value a but different in thevalue r in Eq. (7). The pair of encoder 11 _(n) and decoder 12 _(m) isplaced at one end of a communications system and a pair of encoder 11_(m) and decoder 12 _(n) is placed at the other end of thecommunications system. In accordance with the optical frequency of theoptical code signal that is received by the decoder 12 _(m) of the onecommunications system, the temperature of the planar lightwave circuitsubstrate carrying the decoder is adjusted as described previously withreference to FIG. 15. Since the encoder 11 _(n) and the decoder 12 _(m)are integrated on the same planar lightwave circuit substrate whichundergoes a uniform temperature change throughout it, the opticalfrequencies for filtering by the filters of the encoder 11 _(n) and thedecoder 12 _(m) can be controlled synchronously by the above temperatureadjustment. Accordingly, this temperature adjustment makes it possibleto lower the cross-correlation between the optical frequency of the n-thoptical code signal from the encoder 11 _(n) on the temperature-adjustedside, in this example, and the optical frequency of the m-th decodingcode of the decoder on the non-temperature-adjusted side. As in the caseof FIG. 9 wherein the two encoders 11 _(n) and 11 _(m) are formed on thesame circuit substrate 46, the encoders, which encode optical codesignals the cross-correlation between which degrades upon occurrence ofindividual temperature changes of the encoders, are integrated on thesame planar lightwave circuit substrate which undergoes a uniformtemperature change throughout it—this suppresses deterioration of thecross-correlation. The temperature of the planar lightwave circuitsubstrate may be controlled through utilization of the intensity of thelight transmitted through the encoder. In the case of using two encoderseach of which outputs an optical signal of an opticalintensity-frequency characteristic function Cm(f) for the one value ofbinary data and an optical signal of an optical intensity-frequencycharacteristic function (1−Cm(f)) for the other value, theabove-mentioned temperature adjustment may preferably be made inaccordance with the intensity difference between the transmitted lightoutputs from the both encoders. Furthermore, in the case of using theFIG. 9 structure, the maximum number of system users decreases by halfsince each user is assigned two encoding codes. With the FIG. 28structure, however, because of the combined use of the decoder and theencoder, the maximum number of system users does not decrease by halfdespite the use of the encoding scheme free from interference byreflected light. Though described above as using the same encoded codeas that used in Embodiment 1-2, this embodiment can similarly be appliedto the Hadamard optical code signals shifted one chip apart inEmbodiment 1-3 since the one-chip shift can be suppressed.

Now consider such an optical communications system as shown in FIG. 29,which comprises: a plurality of local office devices; a plurality ofoptical fibers 13 _(A), 13 _(B) and 13 _(C) for transmittingtherethrough signals from the local office devices; a line concentratingcircuit 99 for concentrating the signals from the optical fibers 13_(A), 13 _(B) and 13 _(C) into a single optical fiber 13; and a centraloffice device Cst that receives the concentrated optical signal from theconcentrating circuit 99 via the single optical fiber 13. In such asystem configuration encoders of the respective local offices aredisposed in the concentrating circuit 99 for concentrating the opticalsignals from the multiple optical fibers into the single fiber 13. Thatis, this is the same configuration as that depicted in FIG. 3( a). Sincethe optical signals from all the local office devices can thus besubjected to the same wavelength dispersion irrespective of thedistances between the local office devices and the central officedevice, the same dispersion compensation can be implemented for theoptical code signals from the encoders of the respective local officedevices. Accordingly, dispersion compensation can be provided for theoptical code signals from all the encoders by a single dispersioncompensator which makes compensation to level off the opticalfrequency-dependent delay of the optical fiber 13 connecting theconcentrating circuit 99 to the central office device. Because of thecentralized disposition of the encoders, the encoding opticalfrequencies of all the encoders can easily be adjusted at one location.

In the above description, the optical signal from the light source 10 isinput to the encoder 11 and is added with the optical frequencycharacteristic Cn(f) according to the optical filtering frequencycharacteristic corresponding to the encoding code Cn(f) of the encoder11, and then the optical signal is turned ON and OFF according to themark and the space to generate the n-th optical code signal; however, aswill be described later on with reference to Embodiment 2-8 of thesecond mode of working of the present invention, it is also possible touse chip light sources each of which corresponds to one chip forming theencoding code, for example, outputs an optical signal of a singleoptical frequency. In this instance, those of the chip light sourcescorresponding to the chips “1” forming the encoded code Cn(f) outputoptical signals but those chip light sources corresponding to the chips“0” do not output optical signals; that is, the chip light sourcesresponds to the encoding code Cn(f) to provide the optical signals forthe mark but not to provide the optical signals for the space.

The encoded code in Embodiment 1-3 has such characteristics as describedbelow. When two arbitrary encoding codes are selected from amongdifferent encoding codes of the same code length FCL, they satisfy atleast one of the following conditions:

Number of chip positions in chip strings of the first and secondencoding codes where their corresponding chips simultaneously go to “1s”is equal to the numbers of chip positions where the first encoding codegoes to a “1” and the second encoding code goes to a “−1”; or

Number of chip positions where the first and second encoding codessimultaneously go to “−1s” is equal to the numbers of chip positionswhere the first encoding code goes to a “−1” and the second encodingcode goes to a “1”; and they also satisfy the following condition:

Numbers of chips “1” and chips “−1” in a continuous string of chips ofthe afore-mentioned code length FCL arbitrarily extracted from theconcatenated code of a continuous repetition of the encoding codes areequal to each other irrespective of any particular strings of chips; andthey also satisfy at least one of the following conditions:

Number of chip positions where first and second continuous strings ofchips of the code length FCL, arbitrarily extracted from differentconcatenated codes of continuous repetitions of two different encodingcodes, simultaneously go to “1s” and the chip positions where first chipstring goes to a “1” and the second ship string goes to a “−1” are equalto each other; or

Number of chip positions where the first and second chip stringssimultaneously go to “−1s” and the number of chip positions where thefirst chip string goes to a “−1” and the second chip string goes to a“1” are equal to each other. And the chips forming the encoding code aresequentially assigned consecutive optical frequencies corresponding tothe chip strings.

The source optical frequency width FSR is a natural-number multiple ofthe code length FCL of each encoding code Cn(f), and the opticalfrequency region 31 for encoding by each encoder 11 _(n) the opticalfrequency region 32 for decoding by each decoder 12 _(n) are both withinthe optical frequency range from Fst to Fla, where Fla−Fst>FSR. And itis evident that Cn(f)=Cn(f+FCL) holds in FSR in the range from Fla toFst and that Eq. (13) holds between the code Cn(f) and its complementarycode (1−Cn(f)) as follows:∫Cn(f)·Cn(f)df>∫Cn(f)·(1−Cn(f))df  (14)where ∫df represents a definite integral with respect to f in anarbitrary interval FSR in the optical frequency range from Fst to Fla.

It is also clear that Cn(f) bears the relationship of the followingequation (15) to the encoding code Cm(f).∫Cn(f)·Cm(f)df=∫Cn(f)·(1−Cm(f))df  (15)

In Embodiment 1-2, as is evident from Eq. (7), it is possible to use,for each Cn(f), r′ (typically, r′=2) encoded codes with a=n.

[Second Mode of Working] (Optical Phase·Amplitude Modulation)

The second mode of working of the present invention is intended as asolution to the problems of the prior art through phase modulation andphase or amplitude modulation of a carrier on the optical frequencyaxis.

Embodiment 2-1

FIG. 30 illustrates an optical communications system according toEmbodiment 2-1 of the second mode of working of the invention. In anoptical transmitter 100 a transmission signal from an input terminal101, usually a binary data sequence signal, is converted by asignal-phase converter 110 into a phase shift (phase shift value)sequence or modulation phase sequence (hereinafter referred to also as amodulation unit sequence), where the value of phase shift correspondingto every V pieces of data (where V is an integer equal to or greaterthan 1) is less than one period. An optical signal from a light source120 is fed to a phase modulation part 130, wherein the phase of apseudo-carrier on the optical frequency (wavelength) axis starting at apredetermined optical frequency (wavelength) as a reference is shiftedto each phase amount from the signal-phase converter 110. Thepseudo-carrier will sometimes be referred to simply as a carrier.

The term “pseudo-carrier” corresponds to the term “optical frequencycharacteristic (function),” “encoded code or decoded code,” or “opticalfiltering frequency characteristic (function)” in other modes ofworking.” Since in the second mode of working of the invention thepseudo-carrier on the optical frequency axis is subjected to amodulation similar to QPSK or QAM modulation for the carrier on the timeaxis used in radio communications, the term “pseudo-carrier” will beused primarily in the interests of better understanding of theinvention.

For example, as shown in FIG. 31, the data sequence is divided into datasets each consisting of V=2 pieces of data, and the data sets are eachassigned a different phase shift amount, that is, a phase amount shiftedfrom a reference phase 0 (which will hereinafter refer to the phaseshift amount). For example, data sets (0, 0), (0, 1), (1, 0) and (1, 1)are converted to phase amounts 0, π/2, π, 3π/2, respectively, which areless than one period.

Assume that an optical frequency difference from the reference opticalfrequency fs is a phase f, 400 GHz is one period Λ, and an opticalsignal that is coded by a function which is obtained, by adding 1 to anddividing by 2, a trigonometric function that means the optical intensityof each optical frequency signal, is the pseudo-carrier. When the phasef is 0, π/2, π, and 3π/2, the optical frequency characteristics becomeas shown in FIGS. 31( a) to 31(d), respectively; that is, for each π/2shift of the phase f, the optical frequency shifts by 100 GHz. In FIGS.31( a) to 31(d) the leftmost diagrams each show the pseudo-carrier interms of vector on a complex plane, the second from the left theinstantaneous phase-intensity characteristic of the pseudo-carrier andthe leftmost its optical intensity-frequency characteristic.

Letting an n-multiple of the period Λ of the pseudo-carrier berepresented by FSR, that is, FSR=nΛ (where n=1, 2, . . . ), the lightsource 120 outputs an optical signal of at least the optical frequencywidth FSR. The output optical signal from the phase modulation part 130is ((1+cos(2πfn/FSR+Θ)))/2 whose phase shift amount is any one of Θ=0,π/2, π and 3π/2.

In an optical receiver 200 the received light is split by a splitter 210for input to four filters 221, 222, 223 and 224 corresponding to thephase shift amounts 0, π/2, π and 3π/2, respectively, the intensity oflight transmitted through the filters 221, . . . , 224 is detected bydetectors 231, . . . , 234. The outputs from the detectors 231, 233 andthe outputs from the detectors 232, 234 having detected output lightintensities of the filters corresponding to the phase shift amountsdisplaced one-half period apart, respectively, are subtracted from eachother by comparators 241 and 242, respectively. The outputs from thecomparators 241 and 242 are converted by a code-signal converter 250 toa data set corresponding to the phase shift amount of thepseudo-carrier, which is output as a decoded data sequence.

FIGS. 32-1 to 32-4 show examples of: the wavelength characteristic ofthe light source 120 and the intensity-time characteristic of the lightsource in the case of using a pulse light source; the optical outputfrom the modulation part 130 corresponding to each shift amount Θ (theoptical output from the optical transmitter 100); the filteringcharacteristic of each filter of the receiver 200; thetransmission-intensity characteristic of each filter for each phaseshift amount; and temporal variations of the detected intensity of eachdetector. The leftmost column of each diagram shows the output lightfrom the light source 120; it is assumed, in this case, that the opticalfrequency width of the output light is 400 GHz and that the outputoptical intensity is flat over its entire wavelength range. The columnssecond from the left in FIGS. 32-1 to 32-4 show optical frequencycharacteristics of the output light from the phase modulation part 130corresponding to the phase shift amounts 0, π/2, π and 3π/2,respectively. The columns third from the left show filteringcharacteristics of the filters 221, 223, 222 and 224 of the opticalreceiver, respectively. The columns fourth from the left in FIGS. 32-1to 32-4 show frequency characteristics of transmitted light through thefilters 221, 223, 222 and 224 in the cases where they receive theoptical outputs from the modulator depicted in FIGS. 32-1 to 32-4,respectively. The rightmost columns in FIGS. 32-1 to 32-4 show temporalvariations of the intensity detected by the detectors 231, . . . , 234,respectively. As shown in FIGS. 32-1 to 32-4, letting the power of thesource output light be represented by 1, the detected intensity of theoutput from the filter, which has a filtering characteristic identicalwith the optical frequency characteristic of the modulator output lightcorresponding to the amount of phase shift from the reference in themodulation part 130, is 0.375 (first row in FIG. 32-1 and second row inFIG. 32-2), and the detected intensity of the output from the filterwhose filtering characteristic is displaced π apart from the filteringcharacteristic of the above-mentioned filter is 0.125 (third row in FIG.32-1 and fourth row in FIG. 32-4), and the output from the comparatorthat compares the detected intensities is 0.25. On the other hand, thedetected intensities of the outputs from the filters, whose filteringcharacteristics are displaced π/2 and 2π/3 apart from the frequencycharacteristic of the modulator output light corresponding to the phaseshift amount relative to the reference in the modulation part 130, are0.25 (second and fourth rows in FIG. 32-1 and third and first rows inFIG. 32-2); consequently, the comparator which compares theintensity-detected outputs provides an output 0.

A description will be given below of the output intensity of thecomparator in the case of comparing intensities of optical pulses of thetransmitted light through the filters, not in the case where thedetectors detect transmitted light intensities for each opticalfrequency component and the comparators compare the detectedintensities.

The output light from the phase modulation part 130 is given by thefollowing equation.(½π) ∫((1+cos(2πf/FSR+Θ))/2)df  (16)where f represents an optical frequency difference from a referenceoptical frequency, and n=1.

(1) In the case of the comparator output corresponding to the filterswhich have a filtering characteristic identical with the opticalfrequency characteristic of the phase modulator output lightcorresponding to the phase shift amount of the input to the phasemodulation part 130:(½π)∫((1+cos(2πf/FSR+Θ))/2)((1+cos(2πf/FSR+Θ))/2)df−(½π)∫((1+cos(2πf/FSR+Θ))/2)((1+cos(2πf/FSR+Θ+π))/2)df=⅛π)∫(1+cos2(2πf/FSR+Θ))+2 cos(2πf/FSR+Θ))df=0.25  (17)

The first and second terms on the left side correspond, for example, tothe output from the detector 231 and the output from the detector 233,respectively.

(2) In the case of the comparator output corresponding to the filterswhich have a filtering characteristic phase π/2 apart from the opticalfrequency characteristic of the phase modulator output lightcorresponding to the phase shift amount of the input to the phasemodulation part 130:(½π)∫((1+cos(2πf/FSR+Θ))/2)((1+cos(2πf/FSR+Θ+π/2))/2)df  (18)−(½π)∫((1+cos(2πf/FSR+Θ))/2)((1+cos(2πf/FSR+Θ−π/2))/2)df=(⅛π)∫(−2sin(2πf/FSR+Θ)+sin 2(2πf/FSR+Θ))df=0  (19)

In this embodiment the number M of the phase shift amounts (phase shiftvalues) possible for the pseudo-carrier is an even number 4, and sincethese phase shift amounts (phase shift values) are sequentiallydisplaced π/2 apart, the receiving side uses filters of the same numberas M, but in the case where the phase shift amounts (values) that thepseudo-carrier is allowed to take are not displaced π apart, thereceiving side uses filters of the same characteristics as those of theoutput light of the phase shift amounts (values) that the pseudo-carrieris allowed to take and filters of characteristics phased the half period(π) apart from the output light of the phase shift amounts (values) thatthe pseudo-carrier is allowed to take. Accordingly, the optical receiver200 requires 2M filters and comparators of the same number as M. In thisinstance, the value M is arbitrary, but the phase shift amounts (phaseshift values) of the carrier by the phase modulation part 130 needs tobe phase shift amounts (phase shift values) which differ in theremainder Δ of the trigonometric function over one period for each ofthem.

This embodiment has been described on the assumption that the opticalfrequency characteristic of the output light from the light source 120is flat, but when it is not flat, for example, Gaussian, thetransmitting-side phase modulation part or the receiving-side filters,comparators or detectors need only to assign weights to their outputs soas to level off intensity at each optical frequency.

While in this embodiment the invention has been described above as usinga combination of the single optical transmitter 100 and the opticalreceiver 200, the invention is applicable as well to the case whereother optical transmitter and optical receiver share the same opticaltransmission line 300, in which case the following settings are made.

1) Where the reference optical frequency fs in the optical transmitteris displaced FSR or more: FSR and the phase shift amounts (phase shiftvalues) are both arbitrary.

2) Where the reference optical frequency fs in the optical transmitteris displaced less than FSR: The optical frequency of the light source120 that is used in the same FSR is the same, and the filters ought tocontinuously filter not only the optical frequency of the opticaltransmitter own FSR but also the optical frequencies used by the otheroptical transmitters which share the same optical transmission line. Thevalue, FSR/n, of the carrier period is different for each opticaltransmitter. When the period FSR/n is common to all the opticaltransmitters, letting the number of phase shift amounts (shift values)for modulation by the phase modulation part 130 be represented by Minclusive of the 0-phase, the value of addition of FSR/n/M to thereference optical frequency fs ought to be unique to every opticaltransmitter. However, when different sets of optical transmitters andreceivers use the same value n, only those optical frequency signalsdisplaced a quarter period apart are orthogonal to each other, andconsequently, M is 4, whereas those optical frequency signals displaceda half period apart are used by the optical transmitter and receiver ofthe same set.

In the case of 1), when the reference optical frequencies fs for use inmultiple optical transmitters are displaced FSR or more apart, differentoptical frequencies are used, so no interference occurs betweencarriers, irrespective of the pseudo-carrier.

In the case of 2), when the multiple optical transmitters use the samereference optical frequency fs, they use the same optical frequency.However, since the frequency of the pseudo-carrier used by each opticaltransmitter is a natural-number fraction of FSR, the integration valueof their scalar product over an interval FSR is zero and they areorthogonal to each other, incurring no interference between thecarriers.

When the multiple optical transmitters use different reference opticalfrequencies fs displaced less than FSR apart, since the functions of thecarriers are periodic functions within FSR, use is made of filters whichmodulate optical frequencies outside FSR with the same function as thatfor the pseudo-carrier, and if the optical frequencies of light sourcesare substantially equal, the integration value of their scalar productis zero and they are orthogonal to each other, incurring no interferencebetween the carriers.

The phase modulation part 130 for use in this embodiment comprises, asshown in FIG. 30, a filter 131 and a modulator 132 for changing itsfiltering characteristic.

FIG. 33 illustrates an example of the configuration of the filter 131. AMach-Zehnder interferometer is used which divides the input light by acoupler 131 a into two optical signals for input to two optical paths131 b and 131 c having a predetermined path length difference andcombines the optical signals again by a coupler 131 d, and the modulator132 is one that is inserted in the one optical path 131 b of theMach-Zehnder interferometer to change the path length difference betweenthe two optical paths. The optical signals having passed through theoptical paths 131 b and 131 c interfere with each other in the coupler131 d, which outputs components spaced apart the optical frequencycorresponding to the path difference. The optical frequencycharacteristic of the output light is periodic; therefore, assuming thatthe frequency difference between the optical outputs is 100 GHz, forinstance, there will be obtained the filtering characteristic shown inthe rightmost column of FIG. 31( a). With the use of the filter 131which periodically filters optical frequencies like Mach-Zehnderinterferometer, it is possible to eliminate interference even ifmultiple optical transmitters use different reference opticalfrequencies fs displaced each other less than FSR apart.

To change the path length, the optical paths are integrated on a planarlightwave circuit substrate as shown in FIG. 9, and a phase shift amountsignal from the signal-phase converter 110 is applied to the electrode49 to change the delay amount of the optical path associated with theelectrode. In this instance, the filter 131 and the modulator 132 arenot series-connected but instead they are integrally formed with eachother. That is, the modulator 132 is incorporated into the filter 131.

The signal-phase converter 110 in the optical transmitter 100 isconfigured, for example, as shown in FIG. 30, in which the data sequencefrom the input terminal 101 is converted by a serial-parallel converter110 a to two sequences, the two pieces of data of the two data sequencesfrom the serial-parallel converter 110 a are converted by a D/Aconverter 110 b to digital values 0, 1, 2 and 3 corresponding tocombinations of the input data (0, 0), (0, 1), (1, 0) and (1, 1),respectively, and voltages corresponding to these values are applied tothe electrode 49 in FIG. 9. In accordance with the applied voltagevalues the phase of the pseudo-carrier of the output light from thephase modulation part 130 varies as shown in FIGS. 31( a) to 31(d).

The filter 131 in the optical transmitter 100 filters light from thelight source 120 at least over the optical frequency width FSR, and thefiltering characteristic, that is, the transmittance (opticalintensity)-optical frequency characteristic is such that when theoptical frequency difference from the reference optical frequency fs isused as phase, the transmittance (optical intensity) in each phaseconforms to a function obtained, by adding 1 to and dividing by 2, atrigonometric function indicating the period obtained by dividing FSR bya natural number n.

A code signal converter 250 in the optical receiver 200 is configured,for example, as depicted in FIG. 1, in which outputs 0 or 1 from thecomparators 241 and 242 are input in parallel to a parallel-serialconverter 251, from which they are output as a single sequence of datasignals to an output terminal 201. Thus, the transmission signal inputto the input terminal 101 of the optical transmitter 100 is regeneratedand output to the output terminal 201.

As described above, according to Embodiment 2-1, in order to emulatepositive- or negative-polarized uncorrelated carriers by non-polarizedintensity modulation which is a part of a repetition of a desiredfrequency period on the optical frequency axis, use is made of broadbandlight of an optical frequency width which is a natural-number ofmultiple of the pseudo-carrier period and a differential detection isconducted on the receiving side to inhibit the input thereto ofpseudo-carriers other than those to be received; thus, even if light ofthe same optical frequency is used, the correlation betweenpseudo-carriers is eliminated which is attributable to trigonometricfunctions that are not orthogonal to each other in a finite opticalfrequency width, the uncorrelated pseudo-carriers are emulated, and theemulated pseudo-carriers are phase-modulated—this permits implementationof MPSK with control accuracy lower than that on the order of thewavelength of light. It will be described later on that the second modeof working of the invention is basically common in technical idea to thefirst mode of working.

Embodiment 2-2

In Embodiment 2-1, the modulator 132 of the phase modulation part 130controls the phase of the pseudo-carrier of light filtered or to befiltered by the filter 131. According to Embodiment 2-2, for example, asshown in FIG. 34, the phase modulator is provided with a plurality offilters 133 a, 133 b, 133 c and 133 d each of which filters the opticalfrequency corresponding to the phase shift amount (value) that is theamount of modulation, and the light from the light source 120 is inputvia a splitter 134 to the filters 133 a to 133 d. The optical outputsfrom the filters 133 a to 133 d are selected and output by the modulator132 in accordance with the modulation phase amount (value). For example,the filters 133 a to 133 d have the filtering characteristics that thecarrier phases are 0, π/2, π, and 3 π/2 as shown in FIGS. 31( a) to31(d), respectively, and the optical outputs from the filters 133 a to133 d are input to optical switches 135 a to 135 d which form themodulator 132. In the signal-phase converter 110, the two pieces of dataoutput from the serial-parallel converter 110 a are decoded by a decoder110 c, which provides outputs at its output terminals 111 a, 111 b, 111c and 111 d corresponding to the data sets (0, 0), (0, 1), (1, 0) and(1, 1), respectively, and by the output at each of the output terminals111 a to 111 d the corresponding one of the switches 135 a to 135 d isturned ON. The optical outputs from the switches 135 a to 135 d areprovided via a combiner 136 to the transmission line 300.

As indicated by the broken line in FIG. 34, the switches 135 a to 135 dof the modulator 132 may be disposed between the splitter 134 and thefilters 133 a to 133 d to combine the transmitted outputs from thefilters 133 a to 133 d by the combiner 136. Both the splitting stage andthe combining stage may be incorporated into the modulator 132 whichinputs the light from the light source only to a selected one of thefilters and transmits the light from the selected filter.

Unlike Embodiment 2-1, Embodiment 2-2 eliminates the need for selectingthe material which allows switching of the optical path lengthdifference in a time to modulate. Incidentally, the optical receiver 200may be one that is shown in FIG. 30. While in the above this embodimenthas been described in connection with the case where the number M ofphase shift amounts (values) possible for the pseudo-carrier is 4, thenumber M may be arbitrary.

Embodiment 2-3

In Embodiment 2-3 a set of two pseudo-carriers displaced π apart inphase and a different set of pseudo-carriers displaced π/2 apart inphase are made to correspond to two sets of data of the transmissionsignal. FIG. 35( a) shows an example of the optical transmitteraccording to Embodiment 2-3. The conditions for the light source 120 andthe filters are the same as those in Embodiments 2-1 and 2-2. Thefiltering characteristics of two filters of each set are displaced πapart in phase, that is, their transmitted optical frequencies aredisplaced FSR/2/n apart. Letting the pseudo-carrier for use in thisembodiment be identified as an i-th carrier and setting 2πf=Θ,characteristic functions of the respective sets are Ci(Θ) or Ci(Θ+π) andCi(Θ+π/2) or Ci(Θ+3π/2), and assuming that ∫dΘ is a definite integralover the interval FSR, the following equations hold.∫Ci(Θ)(Ci(Θ+π/2)−Ci′(Θ+π/2))dΘ=∫Ci(Θ)(Ci(Θ+3π/2)−Ci′(Θ+π/2))dΘ=0  (20)∫Ci(Θ)(Ci(Θ)−Ci(Θ))dΘ=∫Ci(Θ+π(Ci(Θ+π)−Ci′(Θ+π))dΘ  (21)

FIG. 36( a) shows QPSK signal points (coordinate points) on a complexcoordinates, and FIG. 36( b) shows, by way of example, signal data sets,coordinate points and sets of select filtering phases in the case ofemulating QPSK. In this instance, the number of sets M/2=2, the phaseshifts of the one data set are 0 and π, and the phase shifts of theother data set are π2 and 3π/2, and their coordinate points are shown ona unit circle in FIG. 36( a). 0 and π in the data set of phase shifts 0and π correspond to 1 and −1 on the x-axis, respectively, whereas π/2and 3π/2 in the data set of phase shifts π/2 and 3π/2 correspond to 1and −1 on the y-axis, respectively. The coordinate points are indicatedin the parentheses with the x-axis values at the left-hand side and they-axis values at the right-hand side.

The phase modulation part 130 outputs light of the pseudo-carrier of the0-phase shift or π-phase shift corresponding to one bit of the two-bitdata set, in the example of FIG. 36( b), the high-order bit (data) a 0or 1 and outputs light of the pseudo-carrier of the π/2-phase shift or3π/2-phase shift corresponding to the low-order bit (data) a 0 or 1.That is, it can be said that the output from the phase modulation partis an optical code signal indicating the combination codes of the twopieces of data in the data sequence for each modulation unit of thepseudo-carrier light output from the phase modulation part.

In the example of FIG. 35( a), as is the case with FIG. 34, filters 133a to 133 d are provided each corresponding to one of thepseudo-carriers; the 0-phase filter 133 a and the π-phase filter 133 care combined, and the π/2-phase filter 133 b and the 3π/2-phase filter133 d are combined. Switches are provided as modulators 132 a and 132 b,and the switch as the modulator 132 a is controlled by the high-orderbit of the serial-parallel converter 110 a of the signal-phase converter110; the switch as the modulator 132 a is connected to the 0-phasefilter 133 a or π-phase filter 133 c, depending on whether thehigh-order bit (data) is a “0” or “1.” The switch as the modulator 132 bis controlled by the low-order bit (data) of the serial-parallelconverter 110 a in such a manner that the switch is connected to theπ/2-phase filter 133 b or 3π/2-phase filter 133 d, depending on whetherthe low-order bit is a “0” or “1.” The transmitted light through thefilters selectively switched by the switches as the modulators 132 a and132 b is output via the combiner 136 to the optical transmission line300.

That is, the input data sequence from the terminal 101 is applied to theserial-parallel conversion part (hereinafter referred to as a sequenceconverting part) 110 a, wherein it is sequentially separated into firstdata sequence (a low-order bit sequence) and a second data sequence (ahigh-order bit sequence); a modulation part 132 b and a modulation part132 a are controlled according to values of respective pieces of data ofthe first and second separate data sequences to outputs optical codesignals of pseudo-carriers (optical intensity-frequency characteristics)corresponding to the data values, respectively, and these optical codesignals are combined into an output optical code signal.

The modulators 132 a and 132 b may be adapted to select the filter towhich light is input. For example, as shown in FIG. 35( b), the lightemitted from the light source 120 is split by a splitter 134 into two,the one splitter of which is selectively fed into the 0-phase filter 133a or π-phase filter 133 c via a switch serving as a modulator 132 a, andthe other of which is selectively fed into to the π/2-phase modulator132 b or 3π/2-phase modulator 132 d via a switch serving as a modulator132 b, and the optical outputs from the filters 133 a to 133 d areprovided via the combiner 136 to the optical transmission line 300. Onboth of the input and output sides of the filters 133 a to 133 cmodulators may be disposed as switches, permitting the passage of lightfrom the light source through only a selected one of the modulators.

The optical receiver corresponding to this embodiment may be of the sameconfiguration as shown in FIG. 30. In such an instance, however, sincethe comparators 241 and 242 output +1s or −1s, converting parts 241 aand 242 a are used in the optical transmitter 200, as indicated by thebroken lines in FIG. 30, to convert −1s to 0s, which are provided to thecode converter 250. It will be easily be understood that the codeconverter 250 thus provides the same signal sequence as the inputtransmission signal sequence to the optical transmitter 100.

In this way, QPSK can be implemented with a lower degree of controlaccuracy on the order of optical wavelength.

Embodiment 2-4

Embodiment 2-4 emulates 16-QAM by use of pseudo-carriers. In thisembodiment, two sets of pseudo-carriers, each consisting of twopseudo-carriers displaced half a period (π) apart in phase, are used toemulate light from the light source according to 16 kinds of data setsin this example, the pseudo-carriers of the one and the other set arephased a quarter period (π/2) apart and orthogonal to each other, andoptical signals with these four pseudo-carriers of light intensities(amplitudes) having either one of two values are combined andtransmitted.

FIGS. 37-1 and 37-2 illustrate examples of embodiment 2-4. The opticaltransmitter 100 shown in FIG. 37-1 uses four filters 133 a to 133 d forgenerating 0-, π/2-, π- and 3π/2-phase pseudo-carriers. The conditionsfor the light source 120 and the filters 133 a to 133 d are the same asin the case of Embodiment 2-3.

FIG. 38( a) shows signal points (coordinate points) and data sets on thecoordinates (the x-axis representing the real part and the y-axis theimaginary part) in th case of 16QAM, and FIG. 38( b) shows therelationships of phase shifts and light intensities (amplitudes) of thepseudo-carriers to the respective data sets. For example, in the casewhere the data set is (0000), light of a pseudo-carrier of 0-phase shiftand optical intensity 3 and light of π/2-phase shift and intensity 3 areoutput to the transmission line 300, and in the case of the data set(0101), light of a pseudo-carrier of 0-phase shift and optical intensity3 and light of 3π/2-phase shift and intensity 1 are output to thetransmission line 300. The transmission signal sequence from theterminal 101 is converted every four data sets by a signal-to-phase andamplitude converter 140 to the phase and amplitude informationindicating the phase shift and the amplitude in FIG. 38( b), and thelight from the light source 120 is modulated by a phase and amplitudemodulating part 150 to two optical signals of the pseudo-carrier phasesand intensities corresponding to the above-mentioned phase and amplitudeinformation, which optical signals are output to the opticaltransmission line 300.

In the signal-to-phase and amplitude converter 140, the inputtransmission signal sequence is divided, for example, by aserial-parallel converter 110 c, into four signal sequences. Thephase-amplitude modulating part 150 is, in this embodiment, formed byfilters 133 a, 133 b, 133 c and 133 d for the pseudo-carriers of phaseshifts 0, π/2, π, 3π/2, respectively, two modulators 151, 152, and acombiner 136. Upon each output of a 4-bit (data) set consisting of fourbits (data) extracted one by one from the four signal sequences outputfrom the serial-parallel converter 110 b, the switch 151 a in themodulator 151 is controlled by the high-order (leftmost in FIG. 38( b))data of the 4-data set; the switch 151 a is connected to the 0-phasefilter 133 a or π-phase filter 133 c, depending on whether the data is 0or 1; and the output light from the switch 151 a is provided to thecombiner 136 via an amplitude changing part 151 b wherein the intensityof the output light is controlled by the third high-order data of thedata set to go to a 3 or 1, depending on whether the data is a 0 or 1.In the modulator 152 the switch 152 a is connected to the π/2-phasefilter 133 b or 3π/2-phase filter 133 d, depending on whether the secondhigh-order data of the 4-data set is a 0 or 1, and the output light fromthe switch 152 a is provided to the combiner 136 via an amplitudechanging part 152 b wherein the intensity of the output light iscontrolled by the low-order data of the 4-bit data set to go to a 3 or1, depending on whether the data is a 0 or 3.

That is, the input data sequence from the terminal 101 is sequentiallyseparated by the serial-parallel converting part (hereinafter referredto as a sequence converting part) 110 c into first to fourth separatedata sequences: the phase modulation part 152 a is controlled accordingto the value of each piece of data of the third separate data sequence,the phase modulation part 151 a is controlled according to the value ofeach piece of data of the fourth separate data sequence, the amplitudechanging part 152 b is controlled according to the value of each pieceof data of the first separate data sequence, and the amplitude changingpart 151 b is controlled according to the value of each piece of data ofthe second separate data sequence.

The modulators 151 and 152 may also be inserted between the light source120 and the filters 133 a to 133 d as depicted in FIG. 39. In thisinstance, the light from the light source 120 is split by the splitter134 into two, one of which is input first to the modulator 151, whereinits intensity is controlled by the amplitude changing part 151 b to goto a 3 or 1 according to the third high-order data, and the thusintensity-controlled light is output via the switch 151 a to the 0-phasefilter 133 a or π-phase filter 133 b according to the highest-orderdata. The other split light from the splitter 134 is input first to themodulator 152, wherein its intensity is controlled by the amplitudechanging part 152 b to go to a 3 or 1 according to the lowest-orderdata, and the thus intensity-controlled light is output via the switch152 a to π/2-phase filter 133 b or 3π/2-phase filter according to thesecond high-order data.

It is also possible to intensity-control the optical inputs to the twosets of filters by the amplitude changing parts 151 b and 152 baccording to the third high-order data and the lowest-order data,respectively, and select the filters of the two sets of filters by theswitches 151 a and 152 a according to the highest-order data and thesecond high-order data, respectively. Alternatively, it is possible toinput the light from the light source to either one of the two filtersof each set and intensity-control the optical outputs from the filtersof the two sets by the amplitude changing parts 151 b and 152 b,respectively.

The optical receiver 200 uses, as shown in FIG. 37-2, the same filters221-224, detectors 231 to 234 and comparators 241, 242 as those shown inFIG. 30, but uses, as a substitute for the code signal converter 250, acode signal converter (data generating means) 260 of the type thatconverts the outputs from the comparators 241 and 242 to a set of fourpieces of data corresponding to two levels (intensities) includingpolarities (positive and negative), that is, four levels, and outputsthese data in serial form. In other words, the comparators 241 and 242each outputs any one of 3, 1, −1 and −3 shown in FIG. 38( b); and a dataset shown in FIG. 38( b) is provided corresponding to such a combinationof the outputs.

In such a code signal converter 260, for instance, as depicted in theoptical transmitter of FIG. 37-2, the outputs from the comparators 241and 242 are converted by A/D converters 262 and 262 to 3-bit digitalvalues each containing a sign (code); these 3-bit (a total of 6 bits)digital values are used as addresses to read a conversion memory 265 toobtain therefrom a data set of such four corresponding bits as depictedin FIG. 38( b), and the output data set is converted by aparallel-serial converter (data generating means) 266 to serial data,which is provided to an output terminal 201. Incidentally, let it beassumed that the relationships between the addresses thereto and thedata to be read out thereof are prestored in the conversion memory 265in such a manner as to obtain the relationships between the comparatoroutputs and the data sets shown in FIG. 38( b).

As described above, according to Embodiment 2-4, as is the case withEmbodiment 2-1, in order to emulate positive- or negative-polarizeduncorrelated carriers by non-polarized intensity modulation which is apart of a repetition of a desired frequency period on the opticalfrequency axis, use is made of broadband light of an optical frequencywidth which is a natural-number multiple of the pseudo-carrier periodand a differential detection is conducted on the receiving side toinhibit the input thereto of pseudo-carriers other than those to bereceived; thus, even if light of the same optical frequency is used, thecorrelation between pseudo-carriers is eliminated which is attributableto trigonometric functions that are not orthogonal to each other in afinite optical frequency width, the uncorrelated pseudo-carriers areemulated, then the simulated pseudo-carriers are phase-modulated, thenmultiple pseudo-carriers which are orthogonal to each other areintensity-modulated with the half period and transmitted at the sametime—this permits implementation of QAM with control accuracy lower thanthat on the order of the wavelength of light.

Embodiment 2-5

While the above-described embodiment uses the pseudo-carriers that varyin an analog fashion on the optical frequency axis, but Embodiment 2-5uses pseudo-carriers that are turned on and off in a digital fashion onthe optical frequency axis. The optical transmitter 100 includes, asshown in FIG. 40, the signal phase-amplitude converter 110 and the phasemodulation part 160 made up of the filter 137 which divides the lightfrom the light source 120 into multiple chips on the optical frequency(wavelength) axis and selectively transmits them, and the modulator 132.

Let FSR represent the optical frequency width of the light from thelight source 120.

The filter 137 receives the light from the light source 120, filters thelight from the light source 120 which has the optical frequency widthFSR, then divides on the optical frequency axis the light of thefrequency width FSR from the light source into L (a multiple of 4)chips, and selectively transmits them. The selection of chips that areallowed to pass through the filter is mapped into phase values asdescribed below. Now, let the divider of L/4 be represented by S. FIG.41 shows examples of filtering characteristics.

0-phase filter 137 a: Filter by repeatedly turning ON (pass) consecutive2S chips and OFF (interrupt) the next 2S chips until L is reached.

π/2-phase filter 137 b: Filter by repeatedly turning OFF (interrupt)consecutive S chips, then ON (pass) the next 2S chips, and OFF(interrupt) the next S chip until L is reached.

π-phase filter 137 c: Filter by repeatedly turning OFF (interrupt)consecutive 2S chips and ON (pass) the next 2S chips until L is reached.

3π/2-phase filter 137 d: Filter by repeatedly turning ON (pass)consecutive 2S chips, then OFF (interrupt) the next 2S chips, and OFF(interrupt) the next S chip until L is reached.

In FIG. 41 there are shown filtering frequency characteristics of thefilers 137 a, 137 b, 137 c and 137 d, for example, in the case where L=4and S=1. In FIGS. 41( a), 41(b), 41(c) and 41(d) where FSR/ncorresponding to 4S chips is one period on the optical frequency axisand one-half the period corresponds to 2S chip, i.e., one period is 2π,there are shown filtering characteristics of the filters 137 a, 137 b,137 c and 137 d which permits the passage therethrough of light of 2Schips of π-width shifted to 0-, π/2-, π- and 3π/2-phase positions,respectively. Accordingly, the light having passed through these filters137 a, 137 b, 137 c and 137 d becomes such that a pseudo-carrier of theperiod FSR/n on the optical frequency (wavelength) axis, which is arectangular pattern of the π-width, that is, of a 50% duty ratio, isphase-modulated to 0, π/2, π and 3π/2, respectively.

Embodiment 2-5 enables implementation of the QPSK modulation byassociating two pseudo-carriers of such phases and two data sets. InEmbodiment 2-5, since the filters 137 a, . . . , 137 d are associatedwith 0, . . . , 3π/2, respectively, the two pieces of data from thesignal-to-phase converter 110 are used, as is the case with the opticaltransmitter of FIG. 35( a), to control the modulators 132 a and 132 b toselect the output light from the 0-phase filter 127 a or the outputlight from the π-phase filter 137 c and the output light from theπ/2-phase filter 137 b or the output light from the 3π/2-phase filter137 d, and the optical outputs thus selected are output via the combiner136 to the optical transmission line 300.

As depicted in FIG. 35( b), the modulators 132 a and 132 b may beconnected to the input sides of the filters 137 a to 137 d. Also,modulators may be connected to input and output sides of the filters 137a to 137 d.

The optical receiver 200 in Embodiment 2-5 is identical in constructionwith the optical receiver shown in FIG. 30 except that 0-, π/2-, π- and3π/2-phase filters 225, 226, 227 and 228 of the same filteringcharacteristics as those of the filters 137 a to 137 d, respectively,are used in place of the filters 221 to 224 in the optical receiver 200depicted in FIG. 30. Accordingly, the parts corresponding to those inFIG. 30 are identified by the same reference numerals and the filtersare denoted by parenthesized reference numerals in FIG. 37-1.

FIGS. 42-1 to 42-2 show examples of the optical frequency characteristicof the light source 120 and the optical intensity-time characteristic ofa pulse light source used as the light source, modulation part outputs(transmitter outputs) corresponding to respective phases, filteringcharacteristics of respective filters in the receiver, light havingpassed through the filters of the receiver for transmitted outputs inrespective pseudo-carrier phases; and temporal changes of the intensitydetected by the respective detectors in Embodiment 2-5. In each figure,the leftmost column shows the optical source output. The source outputis shown in terms of an optical frequency difference from the referenceoptical frequency fs, with the optical frequency width of the lightsource 120 assumed as 400 GHz and the intensity of the source outputassumed as being flat over the entire optical frequency range. In FIGS.42-1 to 42-4 the columns second from the left-hand side show modulationpart outputs corresponding to the phase shifts 0, π/2, π, 3π/2,respectively. In the columns third from the left-hand side, thefiltering characteristics (functions) of the filters 225 to 228 in theoptical receiver 200 are shown in first to fourth rows of the columns,respectively. In each of FIGS. 42-1 to 42-4 the column fourth from theleft-hand side shows in its first to fourth rows the optical frequencycharacteristics of light passing through the filters 225 to 228 in thecase where the modulation part outputs shown there are input to thefilters, respectively. The rightmost columns show in their first tofourth rows temporal changes of the intensities to be detected by thedetectors 231 to 234, respectively.

As shown in FIGS. 42-1 to 42-4, assuming that the power at the detectorat the time of all the chips passing through the filters is 1, thedetected intensity corresponding to the filter of the same filteringcharacteristic as the optical frequency characteristic of the modulationpart output is 0.5, and the detected intensity corresponding to thefilter of a filtering characteristic displaced π apart from the opticalfrequency characteristic of the modulation part output is 0; thecomparator that compares these detector intensities provides an outputof 0.5. For example, in FIG. 42-1, when the modulation part output inthe first row is input, the detected intensity of the output light fromthe filter 231 is 0.5 as shown in the first row, and the detectedintensity of the output from the filter 233 is 0 as shown in the thirdrow. Since the detected intensities corresponding to the filters whosefiltering characteristics are displaced π/2 and 3π/2 apart from theoptical frequency characteristic of the modulation part output are both0.25, the comparator that compares the both detected intensitiesprovides an output 0. For example, in FIG. 42-1, the detectedintensities of the optical outputs from the filters 232 and 234 are both0.25 as shown in the second and third rows, respectively.

It is desirable that the transmission characteristic of each chip on theoptical frequency axis be rectangular, but it is shown in a triangularform for easy distinction of individual chips. In this case, however,since the power at the detector at the time of all the chips passingthrough the filters is normalized to 1, Embodiment 2-5 operates asdescribed previously without losing generality, irrespective of whetherthe transmission characteristic is triangular or Gaussian distributionon the optical frequency axis.

In FIG. 40 there is shown only a single combination of the opticaltransmitter 100 and the optical receiver 200, but when other opticaltransmitter and optical receiver share the same optical transmissionline 330 at the same optical frequency, a different value of L ischosen. L is a multiple of 4 corresponding to the phase shift number Mand is a value obtained by dividing the chip number of the opticalfrequency width FSR by an arbitrary integer n. The value S is a valueobtained by dividing L by the phase shift number M, that is, by 4.Letting the number of the phase shift amount be represented by P, P=0,1, 2, 3, and P=0, P=1, P=2 and P=3 correspond to the phase shifts, 0′,π/ 2′, π′, and 3π/2, respectively. That is, 2πP/M (M=4). Every L chipsit is repeated at least n times to provide the transmittance 1 for chipscorresponding to the remainders concerning the value L obtained byadding 1 to L/2 to PS which is obtained by multiplying the number P ofthe phase shift amount by S and the transmittance 0 for the other chips.That is, letting MOD(A, L) represent the remainder of the division of Aby L, the transmittance 1 is provided for chips of the chip numberscorresponding to Q changing from 1 through the above-mentioned n in(Q−1)L+MOD(PS+1, L) to (Q−1)L+MOD(PS+L/2, L), and the transmittance 0 isprovided for the other remaining chips. Since the product of the value Lchosen here and the corresponding value n is constant, an integral ofthe scalar product of the pseudo-carriers for the interval FSR becomeszero by the differential detection at the receiving side, making itpossible to inhibit the input of pseudo-carriers other than those to bereceived.

As examples of the relations of the above-said values L, M, n, S, P andQ, the chips of the transmittance 1 in the case where n=1, M=4, S=6 andL=24 are shown in gray in FIG. 43( a), and the chips of thetransmittance 1 in the case where n=2, M=4, S=3 and L=12 are shown inFIG. 43( b).

In the case where the filters used in Embodiment 2-5 are filters thatfilter pseudo-carriers of optical frequencies outside FSR according tothe same functions as those inside FSR and are based on the periodicfunction of which period is FSR, an integration value of the scalarproduct of the carriers for the interval FSR becomes zero, and thecarriers are orthogonal to each other and hence they do not interferewith each other. The filters for use in Embodiment 2-5 are such as shownin FIG. 16.

As described above, according to Embodiment 2-5, since positive- ornegative-polarized uncorrelated carriers are emulated by non-polarizedintensity modulation which is a part of a repetition of a desiredfrequency period of broadband light on the optical frequency axis, andsince the emulated carriers are phase-modulated, it is possible toimplement QPSK with control accuracy lower than that on the order of thewavelength of light.

Embodiment 2-6

As described above with reference to Embodiment 2-5, the QPSK modulationcan be emulated by the same scheme as shown in FIG. 35 in which thepseudo-carrier is phase-modulated, in π-width-chip blocks, to any one ofthe phase shift positions 0, π/2, π, and 3π/2 on the optical frequency(wavelength) axis. Assuming that the pseudo-carrier for use in thisembodiment is an i-th carrier and that 2πf=Θ, the filteringcharacteristic functions of each set are Ci(Θ) or Ci(Θ+π) and Ci(Θ+π/2)or Ci(Θ+3π/2); letting Σ represent the sum of addition from h=0 toFSR/δΘ−1 over the period FSR where Θ=hδΘ, the following equations hold:ΣCi(Θ)(Ci(Θ+π/2)−Ci′(Θ+π/2))=ΣCi(Θ)(Ci(Θ+3π/2)−Ci′(Θ+3π/2))=0  (22)ΣCi(Θ)(Ci(Θ)−Ci(Θ))=ΣCi(Θ+π)(Ci(Θ+π)−Ci′(Θ+π))  (23)

Eqs. (22) and (23) are formulae for digital processing of integrationsof Eqs. (20) and (21), respectively.

It will easily be understood that the QAM modulation can be emulatedusing the pseudo-carriers shown in Embodiment 2-4 and by the same methodas described previously with reference to FIGS. 37-1 and 37-2. For thisQAM modulation, as parenthesized in FIGS. 37-1 and 37-2, the opticaltransmitter 100 uses filters 137 a to 137 d as substitutes for thefilters 133 a to 133 d, in which case according to two bits of the dataset from the signal-to-phase and amplitude converter 111 the modulator151 selects one of the filters 137 a and 137 c to control the opticalintensity to be either 1 or 3, whereas according to the other two bitsin the data set the modulator 152 selects one of the filters 137 b and137 d to control the optical intensity to be either 1 or 3. In thisinstance, the optical receiver 200 needs only to use filters 225 to 228as substitutes for the filters 221 to 224, and no further modificationsare needed. The positions where to dispose modulators 151 and 152 arethe same as in the case of Embodiment 2-4. Embodiment 2-6 also producesthe same effects as are obtainable with Embodiment 2-6 for the samereasons as in the latter.

Further, the MPSK modulation can be emulated by using the pseudo-carrierused in Embodiment 2-5 and phase-modulating it to any one of M arbitraryphase shifts by a π-width rectangular pattern.

In other words, when the number of phase shifts is M, L which is thenumber of chips for one period is a multiple of M and a multiple of 2and takes a value obtained by dividing the total number of chips formingthe optical frequency width FSR by n. With L=MS, it is repeated at leastn times every L chips to make those chips 1 (which corresponds to theremainders concerning the value L) of the value obtained by adding 1 toL/2 to PS which is obtained by multiplying S and the number P(P=0, . . ., M−1; letting one period be represented by 2π, the phase shift amountis expressed as 2πP/M) of the phase shift amount and to make the otherchips 0. That is, setting that the remainder of the division of A by Lis Mod(A, L), the chips of the numbers corresponding to Q changing from1 through the above-mentioned n in (Q−1)L+Mod(PS+1, L) to(Q−1)L+Mod(PS+L/2, L) are made a 1 and the other chips are made a 0.

Accordingly, letting Ci(Θ) and Ck(Θ) represent the functions ofpseudo-carriers corresponding to i-th and k-th carriers having differentvalues of n (k being a carrier number other than i), respectively, Σrepresent the summation of addition from h=0 to FSR/δΘ−1 over the periodFSR where Θ=hδΘ and P represent a finite value other than zero, thefollowing equation holds:Σ(Ci(Θ)(Ci(Θ)−Ci′(Θ))=P, ΣCk(Θ)(Ci(Θ)−Ci′(Θ))=0  (24)

A description will be given below of an example in which the totalnumber of chips forming the optical frequency width FSR is nL=24 and M=3in this embodiment. Since L is a multiple of M and also a multiple of 2and a measure of 24, L becomes 24 or 12 or 6, n becomes 1, 2, or 4, andS becomes 8, 4, or 2. FIG. 44 shows the case in which (n, L, M, S)=(1,24, 3, 8) and (2, 12, 3, 4). The gray color means chips of thetransmittance 1. In the case of (n, L, M, S)=(1, 24, 3, 8), since n=1,there is only Q=1: for a phase shift 0 (P=0), the first((1−1)24+Mod(0·8+1, 24)=1) chip a to the 12th ((1−1)24+Mod(0·8+24/2,24)=12) chip b have the transmittance 1 as shown in the left-handdiagram of FIG. 44( a); for a phase shift 2π·(⅓) (P=1), the ninth((1−1)24+Mod(1·8+1, 24)=9) chip a to 20th ((1−1)24+Mod(1·8+24/2, 24)=20)chip b have the transmittance 1 as shown in the left-hand diagram ofFIG. 44( c); and, for a phase shift 2π·(⅔) (P=2), the 17th((1−1)24+Mod(2·8+1, 24)=17) chip a to the fourth ((1−1)24+Mod(2·8+24/2,24)=4) chip b have the transmittance 1 as shown in the left-hand diagramof FIG. 44( c). That is, since the maximum chip number is 24, the firstto fourth chips and the 17th to 24th chips go to 1s.

In the case of (n, L, M, S)=(2, 12, 3, 4), since n=2, there are Q=1 andQ=2. As shown in the right-hand diagrams of FIGS. 44( a) to 44(d): inthe case of the phase shift P=0, for Q=1, the first ((1−1)12+Mod(0·4+1,12)=1) chip a to the sixth ((1−1)12+Mod(0·4+12/2, 12)=6) chip b are made1's, and for Q=2, the 13th ((2−1)12+Mod(0·4+1, 12)=13) chip c to the18th ((2−1)12+Mod(0·4+12/2, 12)=18) chip d are made 1's; in the case ofthe phase shift 2π·(⅓) (P=1), for Q=1, the fifth ((1−1)12+Mod(1·4+1,12)=5) chip a to the 10th ((1−1)12+Mod(1·4+12/2, 12)=10) chip b are made1's, and for Q=2, the 17th ((2−1)12+Mod(1·4+1, 12)=17) chip c to the22nd ((2−1)12+Mod(1·4+12/2, 12)=22) chip d are made 1's; and in the caseof the phase shift 2π·(⅔) (P=2), for Q=1, the first to second chips andthe ninth to 12th chips are made 1's, and for Q=2, the 13th to 14thchips are made 1's and the 21st to 24th chips are made 1's. That is, forQ=1, the ninth ((1−1)12+Mod(2·4+1, 12)=9) chip a to the second((1−1)12+Mod(2·4+12/2, 12)=2) chip b are made 1's, and for Q=2, the 21st((2−1)12+Mod(2·4+1, 12)=21) chip c to the 14th ((2−1)12+Mod(2·4+12/2,12)=14) chip d are made 1's. In this instance, for Q=1, the range overwhich the chip 1 can be shifted is from the first to 12th chip position,whereas for Q=2 it is the range from the 13th to 24th chip positions. Inthis embodiment, too, as will be evident from FIG. 44, even if othersignals having different values of n are received, the interferencebetween pseudo-carriers is cancelled at the receiving side, and hencethey can be received independently of each other.

The filter of the optical transmitter according to this embodiment isprovided with three 0-, 2π/3- and 4π/3-phase filters in place of thefour 0-, π-, π/2- and 3π/2-phase filters forming the filter 131 in FIG.2-5. Instead of using the four 0-, π-, π/2- and 3π/2-phase filters andtwo sets of comparators for comparing the outputs from detectorsconnected to the four filters whose phase shift amounts differ in stepsof π, the optical transmitter of this embodiment is provided with thethree 0-, 2π/3- and 4π/3-phase filters, three π-, 5π/3- and π/3-phasefilters, three π-, 5π/3- and π/3-phase filters whose phase shift amountsdiffer by π from them, respectively, and three sets of comparators forcomparing outputs from detectors connected to the filters whose phaseshift amounts differ in steps of π.

As described above, positive- or negative-polarized uncorrelatedcarriers are emulated by non-polarized intensity modulation which is apart of a repetition of a desired frequency period of broadband light onthe optical frequency axis, and the emulated carriers arephase-modulated; by this, it is possible to implement MPSK with controlaccuracy lower than that on the order of the wavelength of light.

Embodiment 2-7

The π-phase filter 133 c (137 c) and 3π/2-phase filter 133 d (137 d) inthe optical transmitter 100 shown in FIG. 37-1 are omitted, the switches151 a and 152 a in the modulators 151 and 152 are omitted accordingly,the 0-phase filter 133 a (137 a) and the π/2-phase filter 133 b (137 b)are connected to the amplitude changing parts 151 b and 152 b in themodulators 151 and 152, respectively, and the signal-to-phase andamplitude converter 111 is replaced with a signal-to-amplitude converter112, that is, with the serial-parallel converter 110 a in thesignal-to-phase converter 110 a shown in FIG. 30, by which the one bit(data) and the other bit (data) of a two-piece data set are made tocorrespond to the modulators 151 and 152 to effect control such that theoptical intensity is a 3 or 1 depending on the bit 0 or 1. In theoptical receiver 200, the code converter 260 provides data 0 or 1,depending on whether the output intensities of the comparators 241 and242 are 3s or 1s, and outputs the pieces of data in serial form.

With such an arrangement, QAM modulation, which has four signal pointsin the first quadrant as depicted in FIG. 38( a), can be performed notonly for the pseudo-carriers based on the trigonometric function on theoptical frequency (wavelength) axis described previously with referenceto Embodiment 2-4 but also for the rectangular pattern pseudo carrierson the optical frequency (wavelength) axis described above withreference to Embodiment 2-6.

Such QAM modulation with four signal points can be achieved as QAMmodulation which has four signal points in any one of the second, thirdand fourth quadrants in FIG. 38( a), by use of two filters selected fromamong combinations of 133 b (137 b) and 133 c (137 c), 133 c (137 c) and133 d (137 d) and 133 a (137 a) and 133 d (137 d). In these cases, whenthe outputs from the comparators 241 and 242 are negative, they areconverted to data 0 or 1, depending on whether their absolute values are3s or 1s.

Embodiment 2-8

Embodiment 2-8 according to the second mode of working of the inventionuses a plurality of light sources each of which emits light of anoptical frequency characteristic function of a different phase.Referring now to FIG. 45, the optical transmitter of Embodiment 2-8 willbe described below in connection with the case where L=4S. In Embodiment2-8, FSR/n on the optical frequency axis, where n=1, that is, FSR is oneperiod, and multiple light sources which provide, in each period,2S-chip optical outputs at the 0-, π/2-, π- and 3π/2-phase shiftpositions on the optical frequency axis, respectively, are used topermit implementation of the QPSK or QAM modulation according toEmbodiment 2-5 or Embodiment 2-6.

The FIG. 45 example uses two sets of light sources each of which emitslight of an S-chip optical frequency and the optical output intensity ofeach light source can be controlled; the illustrated example is providedwith a total of L/S sets of light sources (each set of which is formedby a single broadband light source of a 2S optical frequency width, or2S light sources) which is twice larger than L/2S.

Of the L/S sets of light sources, L/2S sets of light sources, eachemitting light of the S-chip optical frequency, are used to simulate a0-phase or π-phase carrier. That is, as FIG. 46 shows S-chip outputlight of each of the phases 0, π/2, π and 3π/2, the output light from alight source 120 a which outputs S chips of the first continuous opticalfrequency in each period, that is, 0-phase S chips, and the output lightfrom a light source 120 b which outputs the next S chips, that is,π/2-phase S chips, are used to emulate the 0-phase carrier; and theoutput light from a light source 120 c which outputs the next S chips,that is, π-phase S chips, and the output light from a light source 120 dwhich outputs the next S chips, that is, 3π/2-phase S chips, are used toemulate the π-phase carrier. The remaining L/2S light sources are usedto emulate the π/2-phase carrier or 3π/2-phase carrier. That is, theoutput light from a light source 120 e which outputs the S chipssucceeding the first S chips in each period on the optical frequencyaxis, that is, π/2-phase S chips, and the output light from a lightsource 120 f which outputs the next S chips, that is, π-phase S chips,are used to emulate the π/2-phase carrier; and the output light from alight source 120 g which outputs the next S chips, that is, 3π/2-phase Schips, and the output light from a light source 120 h which outputs thenext S chips, that is, 0-phase S chips, are used to emulate the3π/2-phase carrier. These 0-phase, π/2-phase, π-phase and 3π/2-phasecarriers correspond to the transmitted light of P=0, P=1, P=2 and P=3 inFIG. 43( a), respectively.

The above description has been given of the example in which n=1, butwhen n is an integer equal to or greater than 2, the value of Scorresponding to n in the order of optical frequency needs only to beallocated in S-chip blocks to the 0-phase light source, the π/2-phaselight source, the π-phase light source and the 3π/2-phase light source.The value n that can be used is determined by the relation to the chipnumber L that corresponds to one period obtained by dividing the opticalfrequency width FSR by n. In the cases where FSR=24, n=2, L=24/2=12 andS=12/4=3, light of the 0-phase, π/2-phase, π-phase ad 3π/2-phasepseudo-carriers becomes the same as the transmitted light for P=0, P=1,P=2 and P=3 in FIG. 43( b). The value L is a multiple of 4 correspondingto the number M of phase shifts, and the value S is a value obtained bydividing the value L by the number M of phase shifts that is, by 4(L=4S). Every L chips it is repeated at least n times to turn ON (1) thelight sources for chips corresponding to the remainders concerning thevalue L of the value obtained by adding 1 to L/2 to PS which is obtainedby multiplying the number P of the phase shift amount (P=0, 1, 2, 3) byS and turn OFF (0) the light sources for the other chips. That is,letting MOD(A, L) represent the remainder of the division of A by L, thelight sources (two sets in this example) for the chips of the chipnumbers corresponding to Q changing from 1 through the above-mentioned nin (Q−1)L+MOD(PS+1, L) to (Q−1)L+MOD(PS+L/2, L) are turned ON (1) andthe light sources for the other remaining chips are turned OFF (0).

FIG. 45 illustrates the optical transmitter intended to emulate the QAMmodulation shown in FIG. 38( a), in which the transmission signal (data)sequence from the terminal 101 is converted to four sequences by theserial-parallel converter 110C in a signal-to-phase and amplitudeconverter 113. The third significant data (bit) in each 4-bit data set(the data array in the serial-parallel converter 110 c being the same asthe order of bits for each data set shown in FIG. 38( a)) is used tocontrol the switch 153 a; when the data value is 0, a register (drivesignal generating part) 153 b having stored therein the value 3 isconnected to a switch 153 d; when the data value is 1 and a register(drive signal generating part) 153 c having stored therein the value 1is connected to the switch 153 d; the switch 153 d is controlled by themost significant data (bit); when the data value is 0, the switch 153 ais connected to the 0-phase light source 120 a and the π/2-phase lightsource 120 b, and light of the intensity 3 is output from the both lightsources 120 a and 120 b, that is, light of the 0-phase pseudo-carrier isoutput with the intensity 3; and when the most significant data (bit)value is 1, the switch 153 a is connected to the π-phase light source120 c and the 3π/2-phase light source 120 d, and light of the intensity1 is output from the both light sources 120 c and 120 d, that is, lightof the π-phase pseudo-carrier is output with the intensity 1.

A switch 154 a is controlled by the least significant data (bit) in thedata set; when the data value is 0, a register 154 b having storedtherein the value 3 is connected to a switch 154 d; when the data valueis 1, a register 154 c having stored therein the value 1 is connected tothe switch 154 d; the switch 154 d is controlled by the secondsignificant data (bit) of the data set; when the data value is 0, theswitch 154 a is connected to the π/2-phase light source 120 e and theπ-phase light source 120 f, and light of the intensity 3 is output fromthe both light sources 120 e and 120 f, that is, light of the π/2-phasepseudo-carrier is output with the intensity 3; and when the data valueis 1, the witch 154 a is connected to the 3π/2-phase light source 120 gand the 0-phase light source 120 h, and light of the intensity 1 isoutput from the both light sources 120 g and 120 h, that is, light ofthe 3π/2-phase pseudo-carrier is output with the intensity 1.

It will easily be understood that thus the optical transmitter accordingto this embodiment can also output an optical QAM modulated signalsimilar to that obtainable with the optical transmitter 100 ofEmbodiment 2-6 described previously with reference to FIG. 37.Accordingly, the optical receiver 200 for use in this case may be thesame as shown in FIG. 36. In the optical receiver depicted in FIG. 45the light sources need only direct modulation; for example, in the caseof using laser light sources, their direct modulation is performed bycontrolling the magnitude of their drive current—this dispenses with theexpensive phase and amplitude modulation part 150 composed of filtersand modulations such as employed in Embodiment 2-6.

With this embodiment, too, the QPSK modulation can similarly be done asdescribed previously with reference to Embodiment 2-3 by omitting theswitches 153 d and 154 d in FIG. 45, and connecting the registers 153 band 154 b directly to the switches 153 d and 154 d, respectively, tocontrol the switches 153 d and 154 d by each piece of data of thetwo-data set of the signal-to-amplitude modulator 112 in FIG. 6.Further, the QAM modulation described previously with reference toEmbodiment 2-7 can also be achieved by: omitting the switches 153 d and154 d are omitted; connecting the switches 153 a and 154 a to two setsof light sources (a total of four light sources) that output light ofmutually orthogonal pseudo-carriers; and controlling the switches 153 aand 154 a as referred to previously in connection with Embodiment 2-7.Incidentally, in the example in which the switches 153 a and 154 a areomitted and in the example in which the switches 153 d and 154 d areomitted, the serial-parallel converter 110C is substituted with theserial-parallel converter 110 a of the signal-to-phase converter 110 inFIG. 30.

The QAM modulation can also be implemented by use of two light sourceswhose phase shift amounts differ by π/2, for example, the 0-phase shiftlight source and the π/2-phase chip light source. In this instance,since the phase is one-half that in the QAM modulation by the embodimentdescribed above with respect to FIG. 45, doubling the number of stepsfor intensity modulation control in the FIG. 45 embodiment enablessignal transmission to be achieved at the same level as that by theembodiment. Additionally, the number of light sources used can bereduced by half.

It is also possible to perform, by use of multiple light sources, thesame modulation as the MPSK modulation in which the number of phaseshift positions is an arbitrary value M as described previously withrespect to Embodiment 2-6. Referring to FIG. 45, this example will bedescribed in connection with the case where M=4. In this instance, thesignal-to-phase and amplitude converter 113 in FIG. 45 is replaced withthe signal-to-phase converter 110 in FIG. 30. And multiple light sourcesare used which provide, in each period FSR/n, 2S-chip optical outputs atthe 0-, π/2-, π- and 3π/2-phase shift positions on the optical frequencyaxis, respectively. Since M=4, the pseudo-carrier corresponding to eachphase shift is the same as in the embodiment described above withreference to FIG. 45, and in accordance with the output from thesignal-to-phase converter 110: light of the intensity 1 is output fromthe 0-phase chip light source corresponding to the 0-shift amount andthe π/2-phase chip light source; light of the intensity 1 is output fromthe π/2-phase chip light source corresponding to the π/2-shift amountand the π-phase chip light source; light of the intensity 1 is outputfrom the π-phase chip light source corresponding to the π-shift amountand the 3π/2-phase chip light source; or light of the intensity 1 isoutput from the 0-phase chip light source corresponding to 3π/2-shiftamount and the 3π/2-phase chip light source.

In this way, the MPSK modulation can be achieved without using anexpensive phase modulation part composed of filters and modulators.

The FIG. 45 embodiment uses two S-chip light sources so as to outputlight of one pseudo-carrier. The two light sources can be replaced withone 2S-chip light source. In this instance, however, four kinds ofrelatively broadband 2S-chip light sources are used as is evident fromFIG. 45. With such an arrangement as depicted in FIG. 45, however, it istrue that the number of kinds of light sources is four, but the opticalfrequency width of each light source is S-chip, permitting appreciablereduction in the manufacturing cost as compared with that in the case ofusing 2S-chip light sources. The QPSK modulation can also be performedby use of the four kinds of S-chip light sources. In the FIG. 45embodiment, 4n carriers are required for each of the emulation of the0-phase or π-phase carrier and the emulation of the π/2-phase or3π/2-phase carrier, and for the carrier of each phase, two S-chip lightsources are used, that is, the number of light sources used is a totalof 2×4n. The two light sources for outputting carriers at the sameoptical frequency is substituted with one light source which emits lightwith an intensity of the combined output of the carriers 1 or 0. Thatis, in the case of FIG. 45, letting “1” represent those of the 0-phasechip light source, the π/2-phase chip light source, π-phase chip lightsource and the 3π/2-phase chip light source (Each light source is anS-chip light source, but for the sake of brevity, it is referred to as achip light source, omitting “S.”) which emit light of the intensity 1for the carrier of each phase and letting “0” represent the lightsources which do not emit light, the output from each chip light sourceis as listed below.

0-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(1100)

π/2-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(0110)

π-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(0011)

3π/2-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(1001)

Since light of two carriers in orthogonal relationship is output, thoseof these optical outputs which are emitted from the chip light sourcesof the same phase are combined, and hence the intensity of the combinedlight goes to a 2. Accordingly, by pre-doubling the intensity of theoutput light from light source whose optical outputs are combined, it ispossible to output light of four carriers through use of one lightsource for each of four kinds of S-chip light sources. The output fromeach chip light source is as follows:

0-phase carrier+π/2-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(1210)

0-phase carrier+π/2-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(2101)

π-phase carrier+π/2-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(0121)

π-phase carrier+3π/2-phase carrier:

(0-phase chip light source, π/2-phase chip light source, π-phase chiplight source, 3π/2-phase chip light source)=(1012)

Such a reduction of the number of light sources is applicable as well tothe MPSK modulation using multiple light sources as referred topreviously.

In the manner described above, the number of light sources can bereduced by half as compared with that needed in the FIG. 45 embodimentand the step intervals for intensity modulation control can be madelarger than in the afore-mentioned QAM modulation in which the number oflight sources is smaller than in the case of FIG. 45—this produces theeffect of robustness against the influence of noise.

Embodiment 2-9

Embodiment 2-9 includes the above-described embodiments and makes thepseudo-carrier more generalized, and this example is an application tothe QAM modulation. Referring to FIG. 47, an example of thecommunications system according o this embodiment will be describedbelow.

The optical transmitter 100 is provided with a set of i- and i′-thfilters 161 and 161′ and a set of j- and j′-th filters 162 and 162′.These filters 161, 161′ and 162, 162′ perform filtering of at least theoptical frequency width FSR at the optical frequency (wavelength) oflight that the light source 120 emits. Assuming that the opticalfrequency is an optical frequency spaced from the reference opticalfrequency fs (=C/λs, where C is light velocity), that is, normalized bythe reference optical frequency (It can be said that the filter functionis a parameter representing phase.), the value Ci(f) of the filteringcharacteristic function of the i-th filter 161 in the phase f, which isone of the two filters of the first-mentioned set, and the value Ci′(f)of the filtering characteristic function of the i′-th filter 161′ in thephase f, which pairs with the i-th filter 161, are complementary to eachother; hence, they bear the same relation as that of Eq. (4).Ci(f)+Ci′(f)=1  (4)The j-th filter 162 and the j′-th filter 162′ of the other set also beara similar relation. The characteristic functions of the filters 161,161′, 162 and 162′ constitute the afore-mentioned pseudo-carriers.

The optical receiver 200 according to Embodiment 2-9 is identical inconstruction to the optical receiver 200 in FIG. 37 except that thefilters used differ from those used in the latter. The optical receiverof this embodiment uses filters of the same filtering characteristics ofthe filters used in the optical transmitter, that is, an i-th filter 271identical in filtering characteristic to the i-th filter 161, an i′-thfilter 271′ identical in filtering characteristic to the i′-th filter161′, and j-th and j′-th filters 272 and 272′ identical in filteringcharacteristic to the j-th and j′-th filters 162 and 162′, respectively.

With such an arrangement, the comparator 241 compares the detectedoptical intensity corresponding to the transmitted light through thei-th filter 271 and the detected optical intensity of the transmittedlight through the i′-th filter 271′, and, assuming the value Di(f) atthe normalized optical frequency f, the output from the comparator 241is given by Eq. (3) in the first mode of working of the invention.Di(f)=Ci(f)−Ci′(f)  (3)

By the detector 231 respective optical frequency components of thetransmitted light through the i-th filter are detected as the opticalintensity of the transmitted light as a whole. The same goes for theother detectors. Accordingly, the integration value of the scalarproduct of the filtering characteristic function Ci(f) of the i-thfilter 161 of the transmitting side at the normalized optical frequencyf and the filtering characteristic function Di(f) of the i-th filter 271of the receiving side at the normalized optical frequency f over thecontinuous optical frequency range FSR in which to perform filtering bythe i-th filter 271 is a non-zero finite value, and the relation of thefollowing equation (5)′ holds.∫Ci(f)Di(f)df=P  (5)

Eq. (5) corresponds to a generalized version of Eq. (5) shown in thefirst mode of working of the invention.

The integration value of the scalar product of the filteringcharacteristic function Ci(f) of the i-th filter in the phase f and thefiltering characteristic function Dj(f) of an j-th filter other than thei-th one at the normalized optical frequency f over the continuousoptical frequency range FSR contained in the optical frequency rangewhich is filtered by the filter is zero, and Eq. (6) mentioned in thefirst mode of working of the invention holds.∫Ci(f)Dj(f)df=0  (6)

Accordingly, optical components having passed through the j-th filterare not contained in the comparator output which is provided bysubtracting the output of a detector 231′ for detecting the intensity oftransmitted light through the i′-th filter 271′ from the output of thedetector 231 for detecting the intensity of transmitted light throughthe i-th filter 271. Thus, Embodiment 2-9 enables the receiving side tocancel the input from the other pseudo-carriers except the targetpseudo-carrier for receiving by differential detection.

Furthermore, the filtering characteristic function of the i-th filter161 is a periodic function with the optical frequency as a variable, andit is preferable that the transmittance (value) Ci(f) at the normalizedoptical frequency f repeat in the period at intervals of FSRi(=FSR/n)=Λ) so that Eq. (1) mentioned in the first mode of working ofthe invention holds.Ci(f)=Ci(f+FSRi)  (1)

With such an arrangement, the receiving side is allowed to cancel, bydifferential detection, the input from the other pseudo-carriers exceptthe target pseudo-carrier for receiving not depending on the differencesin optical frequency and in the reference optical frequency fs for eachlight source. In this way, Embodiment 2-9 implements QAM with controlaccuracy lower than that on the order of optical wavelength. It will beunderstood that Eqs. (1), (3) to (5)′ hold for both of thepseudo-carriers based on the trigonometric function used in Embodiments2-1 to 2-4 and 2-7 and the chip-structured pseudo-carriers used inEmbodiments 2-5 to 2-8. Incidentally, in the case of the chip-structuredpseudo-carriers, ∫dΘ is replaced with Σ in the equations. Further, itwill be seen that the integral over an interval from an arbitrary valuef to f+FSR in the optical frequency region for filtering by the filteris a value obtained by dividing FSR by 2, allowing that Eq. (2) in thefirst mode of working of the invention also holds.∫Ci(f)df=FSR/2  (2)

The 0-phase and π-phase pseudo-carriers in Embodiments 2-1 to 2-8correspond to the i-th and i′-th pseudo-carriers in Embodiment 2-9,respectively, and the π/2-phase and 3π/2-phase pseudo-carrierscorrespond to the j-th and j′-th pseudo-carriers in Embodiment 2-9,respectively. That is, Embodiment 2-9 indicates general characteristicsof the pseudo-carriers in the present invention, and it can be said thatthe other embodiments are specialized versions of Embodiment 2-9.

Embodiment 12-10

A description will be given below of another example that employs thechip-structured pseudo-carriers. In FIG. 47, the filters 161′ and 162′are omitted and the signal-to-phase and amplitude converter 111 issubstituted with a signal-to-amplitude converter 112 as parenthesized.The other parts remain unchanged but the filtering characteristics ofthe filters 161 and 162 are adjusted as mentioned below.

The number of chips which, upon receiving light from the light source,the i-th filter 161 of the optical transmitter and the j-th filter 162of the optical transmitter other than the i-th filter or of a differentoptical transmitter sharing the same transmission line turn ON (pass) atthe same time in the optical frequency region FSR/n (n=integer equal toor greater than 1) for filtering by the i-th filter 161 and in theoptical frequency region FSR for filtering by the filter 162 and thenumbers of chips which the i-th filter 161 of the optical transmitterturns ON (pass) in its filtering optical frequency region FSR and thej-th filter 162 of the optical transmitter other than the i-th filter orof a different optical transmitter sharing the same transmission lineturns OFF (not pass) in its filtering optical frequency region FSR areequal to each other. In other words, the number of chips which the i-thfilter 161 and the j-th filter 162 turn On at the same time (at the samechip positions) and the number of chips which at the same chippositions, the i-th filter 161 turns ON and the j-th filter 162 turnsOFF are equal to each other.

Accordingly, letting Ci(Θ), where Θ=2πf, represent the value of an i-thcarrier for a chip of a phase Θ corresponding to the wavelengthdifference (frequency difference) from the reference wavelength (thereference frequency), Ci(Θ)=Ci(Θ+FSR/n); letting Ci′(Θ)=1−Ci(Θ),ΣCi(Θ)(Ci(Θ)−Ci′(Θ) (where Σ is the summation of addition from h=0 toFSR/δΘ−1 over the period FSR where Θ=hδΘ) is a finite value; lettingCk(Θ) represent the in-phase-Θ intensity of the function indicating ak-th carrier other than the i-th carrier (where k is the pseudo-carriernumber of other than i), the relation ΣCk(Θ)(Ci(Θ)−Ci′(Θ))=0 (where Σ isthe summation of addition from h=0 to FSR/δΘ−1 over the period FSR whereΘ=hδΘ) holds; letting Cj(Θ) represent the in-phase-Θ intensity of thefunction indicating a j-th carrier, Cj(Θ)=Cj(Θ+FSR/n); lettingCj′(Θ)=1−Cj(Θ), ΣCj(Θ))(Cj(Θ)−Cj′(Θ)) (where Σ is the summation ofaddition from h=0 to FSR/δΘ−1 over the period FSR where Θ=hδΘ) is afinite value; and letting Cm(Θ) the in-phase-Θ intensity of the functionindicating an m-th carrier other than the j-th carrier (where m is thepseudo-carrier number other than i), ΣCm(Θ)(Cj(Θ)−Cj′(Θ)) (where Σ isthe summation of addition from h=0 to FSR/δΘ−1 over the period FSR whereΘ=hδΘ) holds.

The optical receiver 200 used in this embodiment is the same as thatdescribed previously with reference to Embodiment 2-7. In this way, QAMcan be implemented. In Embodiment 2-10, too, equations that have ∫dΘreplaced with Σ in Eqs. (1) to (4) hold. As the filter for use in thisembodiment, it is possible to use a filter which provides transmittance1/transmittance 0, in accordance with the value of one bit forming theHadamard code, for chips of the number derived from the division of thechip number corresponding to FSR/n, for instance, by the code length ofthe Hadamard code. Moreover, in the case of using a filter which filtersoptical frequencies in the region equal to or wider than FSR incorrespondence to a code that is a continuous concatenation of Hadamardcodes, equations that have ∫dΘ replaced with Σ in Eqs. (1) to (5) holdin an arbitrary optical frequency region FSR.

A description will be given below of a concrete example which usesanother code in Embodiment 2-10. A sequence in which filter ON and OFFchips are 1s and 0s, respectively, corresponds to the longest sequencein which L is the code length (period length). For example, in the caseof L=3, the sequence through which the first filter 161 can be made(101). In this instance, the sequences through the other filters (secondand third filters) can be made (011) and (110), respectively, shiftedfrom the sequence through the first filter 161.

The optical receiver 200 is provided with: an i-th filter 271 whichtransmits therethrough light of ON-chips contained in the opticalfrequency filtered by the i-th filter 161 at the transmitting side; ani′-th filter 271′ which transmits therethrough light of OFF-chipscontained in the optical frequency filtered by the i-th filter 161; afirst detector group (231, 132) for detecting the optical intensity oflight transmitted through each of first filters (271, 272); a seconddetector group (233, 234) for detecting the optical intensity of lighttransmitted through each of second filters (271′, 272′); a comparatorgroup (241, 242) for comparing the optical intensities by subtractingthe intensity detected by the second detector from the intensitydetected by the first detector; and an amplitude-signal converter (260)by which a combination of amplitudes modulated by modulators of thecorresponding transmitting station, output from the comparator group, isconverted to the transmission signal.

Consider, as an example of operation, the comparator output intensity inthe case of receiving the signal (101) from the transmitter providedwith the i-th filter 161 by th receiver corresponding thereto and thecomparator output intensity in the case of receiving the signal (011)corresponding to the j-th filter 162. The output from the detector 231,which detects the optical intensity of the output light from the i-thfilter 271 of the receiver, corresponding to the i-th filter 161, whichpermits the passage therethrough of the ON-chip light, is a two-chipcomponent in the signal (101); the chip of the signal (101) which passesthrough the i′-th filter 171′ of (010) filtering characteristics is 0;the output from the detector 233, which detects the optical intensity ofthe output light from the j′-th filter 272′, is 0; and the output fromthe comparator 241 for comparing optical intensities by subtracting theintensity detected by the detector 233 from the intensity detected bythe detector 231 is a two-chip output.

It is only one chip that received light (1019 passes through a j-thfilter 272 for selecting signal light (011) which corresponds to thej-th filter 162 at the transmitting side, the output from the detector232 for detecting the optical intensity of light transmitted through thej-th filter 272 is a one-chip output, the output from the detector 234for detecting the intensity of light transmitted through a j′-th filterfor selecting the light of an OFF-chip of the signal light (011) is aone-chip output, and the output from the comparator 242 for comparingthe intensity detected by the detector 232 and the intensity detected bythe detector 234 by subtracting the latter from the former is a 0-chipoutput.

As described above, this embodiment also enables the receiving side tocancel the input from the other pseudo-carriers except the targetpseudo-carrier for receiving by differential detection as in Embodiment2-7, permitting implementation of QAM with control accuracy lower thanthat on the order of optical wavelength. In the conventional opticalcommunication method of the type that controls the phase of the opticalsignal, it is necessary to control the phase of a single-wavelength(frequency) optical signal with accuracies on the order of tens ofnanometers which is a few tenth of micrometer for the optical wavelengthand hence is sufficiently accurate; at present such phase control isfeasible experimentally, but from the economical point of view itsrealization is difficult, and the optical communications system thatinvolve QPSK, QAM or similar phase modulation has not been put topractical use yet.

According to the second mode of working of the invention, however, it isrelatively easy to perform MPSK or QAM for optical carriers in thefrequency domain.

It will be seen that in Embodiment 2-10, too, equations with ∫dΘreplaced by Σ in Eqs. (1) to (4) holds in a predetermined period FSR.Accordingly, assuming that the pseudo-carrier has a rectangular patternperiodic function composed of the chip of intensity 1 and the chip ofintensity 0 in the region FSR, the number of chips of the i-th and j-thcarriers corresponding to the filtering characteristics of the i-thfilters 161, 271 and the j-th filters 162, 272 that have the intensity 1(or intensity 0) at the same optical frequency positions and the numberof chips of the i-th carrier that have the intensity 1 (or intensity 0)and the intensity 0 (or intensity 1) at the same optical frequencypositions, respectively, are equal to each other. The rectangularpattern periodic function can also be applied to the case of using thelight sources each corresponding to one chip as shown in Embodiment 2-8.

In the QAM modulation in each of the embodiments described above, eitherthe intensity 1 or 3 is chosen by third and fourth parameters,respectively, to express any one of combinations of four pieces of data1 and 0, that is, any one of 16 data combinations, but provision may bemade to provide any one of 17 or more combinations. That is, the opticaltransmitter needs only to effect selective control, in accordance withthe number of combinations of pieces of data 1 and 0 desired to express,so that the light corresponding to the i-th (or i′-th) and j-th (orj′-th) carriers may have the optical intensity of any one ofpredetermined multiple values. The optical receiver needs only tocontrol the code-signal converter 260 to output that one of possiblecombinations of four or more pieces of data 0 or data 1 whichcorresponds to each possible combination for any one of predeterminedmultiple digital values including pieces of polarized information fromthe A/D converters 261 and 262 in FIG. 37-2.

Any one of the predetermined multiple digital values from the A/Dconverters 262 and 264 may sometimes include the polarized informationand sometimes may not. The latter case corresponds to the case in whichsignal points only in one quadrant in FIG. 38( a), for example, in thefirst quadrant as referred to previously in connection with Embodiment2-7, for instance, and the output from the code-signal converter 260 isconverted to one of possible combinations of two pieces of data;accordingly, when the polarized information is contained as a digitalvalue in the outputs from the A/D converters 263 and 264, thecode-signal converter output is a combination of two or more pieces ofdata. Accordingly, it can be said that any one of multiple valuescorresponds, in general, to a combination of two or more pieces of data.

Embodiment 2-11

FIGS. 48-1 and 48-2 illustrate a communications system according toEmbodiment 2-11 of the present invention. In the optical transmitter100, K (K being an integer equal to or greater than 2) opticaltransmitters 100, described previously with respect to Embodiment 2-3depicted in FIG. 35, are accommodated in parallel as indicated by 100 ₁,. . . , 100 _(K) in FIG. 48-1; the transmission signal (data sequence)from the input terminal 101 is converted by a serial-parallel converter170 to K parallel sequences, which are input to the signal-phaseconverters 110 of the optical transmitters 100 ₁, . . . , 100 _(K),respectively. The outputs from the optical transmitters 100 ₁, . . . ,100 _(K) are combined by a combiner 171, from which the multiplexedoutput is provided to the optical transmission line 300.

In the optical receiver 200, K optical transmitters 100, described foruse in Embodiment 2-3, that is, depicted in FIG. 30, are disposed inparallel as indicated by 200 ₁, . . . , 200 _(K) in FIG. 48-2; theoptical signal from the optical transmission line 300 is split by asplitter 270 into K optical signals, which are input to the splitter 210of the optical receivers 200 ₁, . . . , 200 _(K), respectively. Theoutput signals from the optical receivers 200 ₁, . . . , 200 _(K) areconverted by a parallel-serial converter 273 to the originaltransmission signal.

The optical transmitters 100 ₁, . . . , 100 _(K) and the opticalreceivers 200 ₁ . . . , 200 _(K) performs transmission and receptionwithout interference with each other as described previously withreference to Embodiment 2-3.

The optical transmitters 100 ₁, . . . , 100 _(K) and the opticalreceivers 200 ₁, . . . , 200 _(K) in this embodiment employ filters ofthe type that when a predetermined period is used as a reference period,the period of a trigonometric function contained in the filteringcharacteristic of each filter is a period obtained by dividing thereference period by a natural number. That is, the above-mentionedreference period and the period of each filter of the opticaltransmitters 100 ₁, . . . , 100 _(K) correspond to a pair of fundamentaland multiple periods generated by Fourier transform. For example,assuming that the value of the number N for dividing FSR is in the rangeof 1 to K, these FSR, FSR/2, . . . , FSR/K are in the opticaltransmitters 100 ₁, 100 ₂, . . . , 100 _(K), respectively, and theiroptical signals are combined by the combiner 171 into a combined opticalsignal, which is equivalent to a signal subjected to inverse discreteFourier transform processing.

Let the periods of filtering characteristic functions of the filters foruse in the optical receiver 200 be represented. Such relationshipsprovide an operation equivalent to that by which the received opticalsignal is split into optical signals and they arediscrete-Fourier-transformed by the optical receivers 200 ₁, . . . , 200_(K) into the original transmission signal.

In this way, according to Embodiment 2-11, it is possible to implementpseudo OFDM (Orthogonal Frequency Division Multiplex) by use of multiplepseudo-carriers compatible with inverse discrete Fourier transformthrough utilization of the orthogonality between the pseudo-carriers. Inthe case where the optical transmitter 100 uses the optical transmitters100 ₁, . . . , 100 _(K) each of which is provided with filters offiltering characteristics phased π/2 apart for each period from thefundamental period to a period K/2 times the former, if the one of theπ/2-phased-apart filtering characteristics in the optical transmitters100 ₁, . . . , 100 _(K) is a cosine function, the other is a sinefunction; the output from the optical transmitter 100 can be expressedby Σ(an cos((n/FSR)Θ)+bn sin((n/FSR)Θ)), where n is a value of amultiple of the filter period for the fundamental period, and an and bnare transmission signals that are carried by respective pseudo-carriers.

Even if the optical transmitter 100 n and the optical receiver 200 n arenot provided which correspond to an arbitrary period n including thefundamental period, since the value of an or bn concerned is a 0, theequation of the output from the optical transmitter 100 holds, andconsequently the generality of this embodiment is not impaired.

Unlike ordinary WDM (Wavelength Division Multiplex) this embodimentenables wavelengths to be superimposed on each other, and hence does notcalls for the guard band needed in ordinary WDM, providing increasedwavelength utilization efficiency.

Incidentally, the light sources 120 of the optical transmitters 100 ₁, .. . , 100 _(K) may be replaced with a single light source. While theEmbodiment 2-11 uses multiple optical transmitters 100 ₁, . . . , 100_(K) utilizing pseudo-QPSK, it is also possible to use multiple opticaltransmitters utilizing the afore-mentioned pseudo-MPSK or pseudo-QAM.Further, the filtering characteristic function is not limitedspecifically to the trigonometric function but may also be a functionthat has the properties referred to previously with reference toEmbodiment 2-9. Accordingly, it is also possible to use pluralities ofoptical transmitters 100 and optical receivers 200 that use thechip-structured pseudo-carriers described previously with respect toEmbodiments 2-5 to 2-8. In this instance, assuming that S, which is apredetermined measure of L/4, is a reference S, optical transmitters 100₁, . . . , 100 _(K) and optical receivers 200 ₁, . . . , 200 _(K) areused which are provided with filters each having a filteringcharacteristic based on S corresponding to the reference S. That is, thereference S and the filtering characteristic S of each filter of theoptical transmitters 100 ₁, . . . , 100 _(K) correspond to the set ofthe fundamental and multiple periods generated by Fourier transform. Inthis way, the optical transmitter 100 of Embodiment 2-11 also sends theinverse-discrete-Fourier-transformed signal, and the optical receiver200 discrete-Fourier-transforms the received signal into the originaltransmission signal.

The filter for use in the optical transmitter of any embodimentsdescribed above may be a filter adapted to control its filteringcharacteristic by the modulator output as described previously inreference to FIG. 33; alternatively, the filter may be configured toselect a plurality of filters of fixedly set filtering characteristics.Accordingly, to control the filter by the modulator means control of thefiltering characteristic and control of selection of filters.

Since the signal-phase converter 110, the signal-to-phase and amplitudeconverter 111 and the signal-amplitude converter 112 are to convert, inaccordance with signal data, the input thereto to parameters forcontrolling filtering characteristics, selective control of filters andintensity control of light from the optical transmitter, they cangenerically be called signal-to-modulation value converters, and thephase amount and amplitude amount output therefrom can be calledmodulated values, and their respective components can be referred to asparameters.

While the second mode of working of the invention, which performs MPSKor QAM by use of periodic functions on the optical frequency axis asmentioned above, has been described previously in relation to itsgeneral configuration, but it can also be explained as follows. Lettingthe optical frequency width FSRi represent a value obtained by dividingthe optical frequency width FSR, which is the least common multiple, byan integer Ni corresponding to the repetition period of the i-th opticalfrequency characteristic function Ci(f) in the optical frequency widthFSR in the optical frequency range from the optical frequency Fst toFla,Ci(f)=Ci(f+FSRi),∫Ci(f)·Cj(f)df>∫Ci(f)·(1−Ci(f))df,and for the j-th optical frequency characteristic function Ci(f) otherthan the i-th one,∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df;and letting Δf represent the remainder of the division of the periodFSRi of the function Ci(f) by an arbitrary optical frequency width equalto or narrower than Ci(f)FSR and a phase 2π (Δf/FSRi) represent thephase difference from Ci(f),Ci′(f)=Ci(f+Δf),

that is, Ci′(f) is phased 2π(Δf/FSRi) apart from Ci(f), and∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df;an input binary data sequence is sequentially separated into multipleseparate data sequences, and for each data of each separate datasequence in a repeating cyclic order, the phases or/and amplitudes of ani-th optical signal of the i-th optical frequency characteristicfunction and a j-th optical signal of the j-th optical frequencycharacteristic function are controlled in accordance with the value ofsaid each piece of data, and the first and second optical signals thusobtained are combined by one or more light sources and transmittedtherefrom as an optical code signal.

An example of the optical frequency characteristic function is atrigonometric function which has different FSRi and the same FSR butwhose Δf is FSRi/4 or −FSRi/4.

Another example of the function Ci(f) is a function which divides FSRinto a continuous optical frequency portion which is a value L, intowhich FSR is divided by 2SNi which is twice larger than the product ofarbitrary integer S and Ni, and which repeats Ni times making Sconsecutive optical frequencies of each L-long optical frequency portionhave the intensity 1 and the succeeding S optical frequencies have theintensity 0, or which sequentially shifts the positions of the Sconsecutive optical frequencies of the intensity 1 by a predeterminedamount.

The third mode of working of the invention is also applicable to thepoint-to-N point optical communication network PON shown in FIGS. 2( a),2(b), 3(a) and 3(b). It is also possible to apply to the third mode ofworking of the invention the technique of the first mode of working bywhich the i-th encoder on the monolithic planar lightwave circuitsubstrate controls the temperature of the planar lightwave circuitsubstrate which uses transmitted light through any one of the j-thdecoders as described previously with reference to FIGS. 15 and 28.Moreover, the optical encoding method of the first mode of working,which employs the arrayed waveguide AWG, described previously withreference to FIGS. 21 and 22, is also applicable to the opticaltransmitter of the third mode of working.

[Third Mode of Working] [Reflective Optical Communication].

The third mode of working of the present invention is an application ofthe invention to a point-multipoint PON (Passive Optical Network) inwhich multiple subscriber terminals are connected to a central officevia an optical fiber transmission lines, such an optical communicationssystem as shown in FIG. 2 or 3.

Embodiment 3-1

A description will be given, with reference to FIG. 49, of thefunctional configuration of the basic concept of this third mode ofworking. Downstream signal light modulated (encoded) in accordance withbinary data is input to a port 420 a of an optical input/output unit 420via an optical fiber 410 and then via an optical input/output port 412,thereafter being input to a switch 430 via a port 420 b of the opticalinput/output unit 420. The switch 430 is controlled by an upstream datasequence from a terminal 431 to input the downstream signal light to amark encoder 440M or space encoder 440S. The output light from the markencoder 440 and th space encoder 440S is input via an optical combiner450 to a port 420C of the optical input/output unit 420, and the inputlight is output as upstream signal light from the port 420 a of theoptical input/output unit 420 to the optical fiber 410 via the opticalinput/output unit 412. As the optical input/output unit 420 an opticalcirculator is used as indicated by the broken line in FIG. 49, but anoptical directional coupler, an optical coupler/splitter, or the likemay also be used. However, in order to avoid intensity modulation of theupstream signal light by a coherent crosstalk with reflected light andto reduce optical loss, the optical input/output unit 420 may preferablybe an optical circulator. The optical combiner 450 may be formed by anoptical coupler/splitter or switch that is controlled in interconnectedcontrol of the switch 430 as indicated by the broken line; in short, theoptical combiner needs only to input the output light from the markencoder 440M and the output light from the space encoder 440S to theport 420 of the optical input/output unit 420. The switch 430 providesthe input light to the mark encoder 440M or space encoder 440S,depending on whether the data from the terminal 431 is the mark orspace. In the case of using as the optical combiner 450 a switch whichis controlled by the data from the input terminal 431, the switch 430may also be an optical splitter.

The mark encoder 440M and the space encoder 440S are formed, forinstance, as filters which output, over the entire period of the opticalfrequency (wavelength) of the downstream signal light, optical signalsthat are expressed by different functions each having the opticalfrequency (wavelength) as a variable. The optical frequencycharacteristics (optical codes) of the input downstream signal light andthe output upstream signal light are functions that bear suchrelationship as described below.

Let a function of the optical intensity for the optical frequency f whenthe downstream signal light is mark, a function of the optical intensitywhen the downstream signal light is space, a function of the opticalintensity for the optical frequency f when the upstream signal light ismark, and a function of the optical intensity when the upstream signallight is space be represented by IM(f), IS(f), OM(f) and OS(f),respectively. The integration value of the product of the functionsIM(f) and the function OM(f) with respect to f or summation of them andthe integration value of the product of the functions IM(f) and thefunction OS(f) with respect to f or the summation of them are equal, andthe integration value of the functions IS(f) and the function OM(f) withrespect to f or the summation of them and the integration value of theproduct of the functions IS(f) and the function OS(f) with respect to for the summation of them are equal. That is, the functions bear therelationships that satisfy either one of the following equations (22)and (23) and either one of the following equations (24) and (25).∫IM(f)OM(f)df=∫IM(f)OS(f)df  (22)ΣIM(f)OM(f)=ΣIM(f)OS(f)  (23)∫IS(f)OM(f)df=∫IS(f)OS(f)df  (24)ΣIS(f)OM(f)=ΣIS(f)OS(f)  (25)

In the above, ∫df means the above-mentioned integral for an interval ofthe optical frequency period of the downstream signal, and Σ means theabove-mentioned summation for an interval of the optical frequencyperiod of the downstream signal. The digital operations for Eqs. (22)and (24) are similar to the operations for Eqs. (23) and (25).

These relationships indicate that the optical intensity of the markfunction and the optical intensity of the space function are equal toeach other and that components corresponding to a half of the frequencycomponents forming the mark or space downstream signal light can beformed as mark or space upstream signal light. Accordingly, this opticaltransmitter is capable of outputting the upstream signal light modulatedby the same optical power without lowering the modulation degree whetherthe downstream signal light be mark or space.

The above-mentioned functions may be such as shown in FIG. 6. FIGS. 6(a) to (c) show examples of trigonometric functions, which are identicalin amplitude, and overnormalized optical frequency period from 0 to 1which is obtained through the optical frequency period from f0 to fL isnormalized by the reference optical frequency f0=fs, the numbers ofintensity fluctuations contained in this optical frequency period are 1,2 and 3, respectively; and one of the solid lines or broken lines phasedπ/2 apart, or the one-dot-chain line displaced about π/4 from the solidline as shown in FIG. 6( a) is used as the mark function and thefunction displaced π apart therefrom is used as the space function; andfor different directions or different optical communication equipment,another function bearing the relation shown in FIG. 6 is used as themark function and a function phased π apart therefrom is used as thespace function. Alternatively, as shown in FIG. 50, the frequency periodfrom f0 to fL (normalized frequencies 0 to 1) is divided into L chips(optical frequencies), and when the function shown in FIG. 50( a) is,for instance, the mark function, the space function has the same numberof chips of intensity 1 as that of the mark function as shown in FIG.50( b), and a half of the intensity-1 chips of mark or space function ofthe downstream signal light can be used for the upstream signal light.The first half of FIG. 50( b) is the same as the first half of FIG. 50(a), and the second half is an complementary version of the second halfof FIG. 50( a). As an example of each chip the optical intensity isshown in triangular form, but it is desirable that the optical frequencycharacteristic of each chip be in flat rectangular form.

As an encoder in the case of using a trigonometric function as thefiltering function of the mark or space encoder 440M or 440S, it ispossible to use such a Mach-Zehnder interferometer as depicted in FIG. 7which is composed of a pair of optical paths 41 and 42 of different pathlengths and couplers 43 and 44 coupled to both ends of the opticalpaths. FIG. 51 shows an example of the configuration of an encoder forforming, as the filtering function of the mark or space function 440M or440S, such a chip string as shown in FIG. 50. The input light is fed toan optical filter 5, and the optical filter 5 outputs optical frequencysignals of respective chips to different ports and outputs opticalcomponents displaced an integral multiple of the optical frequency ΔFapart. For example, when the output light from the encoder 440M or 440Srepeats the same pattern every four chips, components of opticalfrequencies F₁+qΔF, F₂+qΔF, F₃+qΔF and F₄+qΔF (where q=1, 2, . . . ) areoutput from ports 1, 2, 3 and 4 of the optical filter 5, respectively.Of these outputs, the outputs from the ports corresponding to the chipsof the intensity 1 are coupled by a coupler 6 and output therefrom. Assuch an optical filter 5, AWG (Arrayed Waveguide Graiting) can be usedas is the case with the filter 84 in FIG. 21.

The switch 430 provides the input light to the mark encoder 440M orspace encoder 440S, depending on whether the data from the terminal 431is mark or space. When the optical combiner 450 is formed by a switchthat is controlled by the data from the input terminal 431, the switch430 may be an optical splitter. The upstream signal light and thedownstream signal light may be transmitted over different opticalfibers. For example, as indicated by the broken line in FIG. 49,provision may be made to input the upstream signal light from theoptical combiner 450 to an optical fiber 411, omitting the opticalinput/output unit 420. Alternatively, as depicted in FIG. 52, totalreflectors 451M and 451S are provided for total reflection of theoptical outputs from the mark encoder 440M and the space encoder 440S sothat the optical outputs from the mark encoder 440M and the spaceencoder 440S pass through the mark encoder 440M and the space encoder440S, respectively, and input via the switch 430 to the optical fiber410.

The device configuration of FIG. 52 dispenses with the opticalinput/output unit 420 in FIG. 49 and the optical combiner 450, and hencepermits further reduction of the number of parts used; the number ofparts for modulation is smaller than in the FIG. 49 configuration usinga combination of switches, and as compared with the case of using acombination of a switch and an optical combiner instead of using twoswitches in FIG. 49, the illustrated device configuration produces theeffect of avoiding the optical loss by the optical combiner.

In FIGS. 49 and 52 there is not shown a downstream signal lightreceiving circuit, but it is also possible that at a stage preceding theswitch 430 a portion of the downstream signal light is branched to adownstream signal light receiving circuit to decode the downstream datasequence. Reception and decoding similar to this will be described lateron with reference to FIG. 53, for instance. In the third mode of workingof the invention, the light source is an optical transmitter of thecommunication partner which generates the downstream signal light(optical code signal).

As described above, according to this embodiment, light encoded by oneof the mark and the space equal in optical intensity to each other isreceived and a half of the optical frequencies forming the received markor space downstream signal light is sent back as mark or space upstreamsignal light. Hence, light to be modulated as the upstream signal lightcan be supplied without the need for sending non-modulated CW light tobe modulated as the upstream signal light separately of the downstreamsignal light and without impairing the extinction ratio of thedownstream signal.

With the equipment disclosed in document 3, the central officeseparately sends the downstream signal light for sending its information(data) and non-modulated CW (Continuous Wave) light in the downstreamdirection so that the local office sends it back as an upstream signalmodulated by information (data) of the local office itself. As a result,the downstream signal to be sent back to the central office is notactually used for information transmission therefrom. With the equipmentset forth in document 4, the central office purposely worsens theextinction ratio of the downstream signal light for sending theinformation (data) of the central office itself, and the local officemodulates the received optical signal by the information (data) of thelocal office itself and sends the modulated signal as the upstreamsignal light to the central office; hence inefficient continuous wavelight is used. However, this degrades the extinction ratio of either ofthe downstream signal light from the central office and the upstreamsignal light from the local office, giving rise to the problem ofdeteriorated communication quality.

But the third mode of working of the invention permits modulation of theupstream signal light without the need for transmitting non-modulatedlight and without worsening the extinction ratio of the downstreamsignal light. In the third mode of working of the invention, the encoderis encoding means which modulates, based on its encoding function, theoptical signal so that the optical intensity-frequency characteristic ofthe modulated optical signal becomes an optical intensity-frequencycharacteristic obtained by multiplying the optical intensity-frequencycharacteristic of the downstream signal light by the opticalintensity-frequency characteristic of the encoding function, and thedecoder is decoding means which decodes and outputs, based on itsdecoding function, from the optical signal a component whose opticalintensity-frequency characteristic is the decoding function.

Embodiment 3-2

Embodiment 3-2 is an example in which respective optical frequencycharacteristic functions made to be orthogonal to each other and to bechip codes. Referring to FIG. 53, Embodiment 3-2 will be describedbelow. The downstream signal light from the optical fiber 410 is inputto a downstream mark decoder 464M and a downstream space decoder 461Svia the optical input/output port 412, then the optical input/outputunit 420 and then optical splitters 421 and 422, and light having passedthrough these decoders 461M and 461S is converted by optical detectors470M and 470S into electrical signals; these electrical signals arecompared by a comparator 480 to detect, for example, the differencebetween them, and if the magnitude of the difference exceeds apredetermined value, it is provided as a downstream data sequence to anoutput terminal 481.

The other downstream signal light split by the first optical splitter421 is input to the switch 430, and as shown in FIG. 49, it is modulatedby the upstream data sequence from the input terminal 431 into theupstream signal light, which is fed to the optical input/output unit420, from which it is output via the optical input/output port 412 tothe optical fiber 410. When the optical combiner 450 is formed by aswitch which is controlled by the data from the terminal 431, the switch430 may be substituted with an optical splitter.

In Embodiment 3-1 an integration value of the difference obtained bysubtracting the space upstream signal light from the mark upstreamsignal light and the mark or space downstream signal light, with respectto an optical frequency, or the summation of them is zero; that is, theoptical characteristic functions of them are made to apparently beorthogonal to each other. More specifically, the downstream signal lightis a natural-number NI set of input light which has an optical frequencycharacteristic identical with the optical frequency function of eitherone of the mark and space; letting the optical intensity functions ofthe i-th mark and space be represented by IMi(f) and ISi(f),respectively, the relationship between the i-th downstream signal lightand a j-th downstream signal light other than the i-th one bothcontained in the NI sets satisfies the following equation (26) or (27).∫IMi(f)(IMj(f)−ISj(f))df=∫ISi(f)(IMj(f)−ISj(f))df=∫IMj(f)(IMj(f)−ISi(f))df=∫ISj(f)(IMi(f)−ISj(f))df=0  (26)ΣIMi(f)(IMj(f)−ISj(f))=ΣISi(f)(IMj(f)−ISj(f))=ΣIMj(f)(IMi(f)−ISi(f))=ΣISj(f)(IMi(F)−ISi(f))=0  (27)And, the relationship between the i-th downstream signal light and thei-th upstream signal light contained in the NI set satisfies thefollowing equation (28) or (29).∫IMi(f)(OMi(f)−OSi(f))df=∫ISi(f)(OMi(f)−OSi(f))df=∫OMi(f)(IMi(f)−ISi(f))df=∫OSi(f)(IMi(f)−ISi(f))df=0  (28)ΣIMi(f)(OMi(f)−OSi(f))=ΣISi(f)(OMi(f)−OSi(f))=ΣOMj(f)(IMi(f)−ISi(f))=ΣOSi(f)(IMi(F)−ISi(f))=0  (29)

In the above, ∫ means the above-mentioned integral for an interval ofthe optical frequency period of the downstream signal, and Σ means theabove-mentioned summation over the optical frequency period of thedownstream signal. Incidentally, the downstream signal light to bedetected in the same optical communication equipment and the upstreamsignal light to be output therefrom have different functions. Thedigital operations for Eqs. (26) and (28) are similar to the operationsfor Eqs. (27) and (29).

As the function having such properties as mentioned above, it ispossible to use the Hadamard codes such as shown in FIG. 10. The opticalfrequency characteristic function of the signal light, if used as thefiltering function, is such that, for example, for the mark signallight, the optical frequency chip 1 is allowed to pass but the opticalfrequency chip 0 is not allowed, whereas for the space signal light theoptical frequency chip 1 is not allowed to pass but the opticalfrequency chip 0 is allowed. That is, in the case of the same code, theoptical intensities 1 and 0 are reversed between the mark signal lightand the space signal light. The code 2[0101], the code 3 [0011] and thecode 4[0110] satisfy Eqs. (22) to (29). For example, in order togenerate the signal light of the code 2[0101], the ports of the opticalfilter 5 from which to output optical wavelengths (optical frequencies)λ₂ and λ₄ are connected to the coupler 6 as shown in FIG. 51, the portsfrom which to output optical wavelengths (optical frequencies) λ₁ and λ₃are connected to the port 6′ as indicated by the broken lines, and theoutputs from the couplers 6 and 6′ are selectively output from theswitch 450 serving as an optical combiner, depending on whether the datais mark or space.

Because of such codes, for example, when an i-th signal light is to bereceived but a different signal light, for instance, a j-th one isactually input, the filtering characteristic functions of the downstreammark decoder 461M and the downstream space decoder 461S are IMi(f) andISi(f), respectively, and the optical frequency functions of the inputlight is IMj(f) or ISj(f), and Eqs. (26) or (27) holds; consequently,the difference between the detected optical intensities of the outputsfrom the decoders 461M and 461S is cancelled in the comparator, so thatsignal light other than the i-th one to be received is not provided tothe output terminal 481. Further, the filtering characteristic functionsof the upstream mark encoder 441M and the upstream space encoder 441Sare OMi(f) and OSi(f), respectively, and even if the upstream signal isreflected on the optical transmission line and the reflected light isinput to the downstream mark decoder 461M and 461S, Eq. (28) or (29)holds; accordingly, the detected optical intensities by the opticaldetectors 470M and 470S are cancelled by the comparator 480 and nooutput therefrom is provided to the output terminal 481. In other words,even if reflected light exists, it does not constitute any obstacle tothe reception of signal light of the desired code.

Other examples of functions having such characteristics will bedescribed below. The optical frequency period from f0 to fL (normalizedoptical frequencies) of the downstream signal light is divided into L=4sas shown in FIG. 54( a), for instance; the i-th mark function Mi(f)repeats n=2 times transmission (optical intensity 1) of the first schips and no-transmission (optical intensity 0) of the succeeding schips, and the i-th space function Si(f) repeats n=2 times transmission(optical intensity 1) of the first s chips and non-transmission (opticalintensity 0) of the succeeding s chips. When the period f0 to fL isdivided into L=6s as shown in FIG. 54( b), the i-th mark function Mi(f)repeats n=3 times transmission of the first s chips and non-transmissionof the next s chips, and the i-th space function Si(f) repeats n=3 timestransmission of the first s chips and no-transmission of the next schips. In general, the function mentioned herein is the filteringcharacteristic function (optical intensity-frequency characteristicfunction) represented by the optical frequency of L chips into which theoptical frequency period from f0 to fL is divided, the number obtainedby dividing L by 2 being a multiple of s; letting IMi(f) represent thefiltering characteristic function for the i-th mark and ISi(f) representthe filtering characteristic function for the i-th space, the i-thfiltering characteristic function IMi(f) is a function that repeatstransmission of s chips and non-transmission of the succeeding s chipsat least the number of times (n times) obtained by dividing L by 2s, andthe i-th filtering characteristic function ISi(f) is a function thatrepeats non-transmission of s chips and transmission of the succeeding schips at least the number of times (n times) obtained by dividing L bys. Incidentally, the illustrated functions start transmission ornon-transmission of consecutive s chips at f0, but in such a case asshown in FIG. 54( c) where L=6s and n=3, the function may also be onethat starts transmission or non-transmission of chips of an arbitraryinteger s smaller than s, then repeats non-transmission or transmissionof s chips and transmission or non-transmission of the succeeding schips the number of times obtained by subtracting 1 from a numberobtained by dividing L by 2s, followed by transmission ornon-transmission of (s−s0) chips. That is, despite of theabove-mentioned relationship, the function may be such a function asshown in FIG. 54( c) which is phased apart from the function of FIG. 54(b), for instance. The code 2 of the afore-mentioned second-orderHadamard matrix is L=4, s=1 and n=2, the code 3 is L=4, s=2 and n=1, andthe code 4 is a cyclically left shifted version of the code 3 by thephase π/4. These relationships are the same as the characteristicfunctions shown in Embodiment 2-6 of the second mode of working of theinvention; for example, IMj(f), ISi(f), OMj(f), and DSj(f) correspond toCi(f), (1−Ci(f)), Cj(f) and (1−Cj(f)), respectively.

It will easily be seen that these filtering characteristics, theencoders 441M and 441S and the decoders 461M and 461S which have suchfunctions can be similarly implemented by use of such optical filter 5and couplers 6 and 6′ as shown in FIG. 51. In the case of using theencoder of such a configuration as mentioned above, the upstream markencoder 441M and the upstream space encoder 441S are formed integral asan upstream encoder as indicated by the one-dot-chain line in FIG. 53,the switch 430 is omitted, the optical combiner 450 is formed by aswitch, the downstream mark decoder 461M and the downstream spacedecoder 461S are also formed integral as a downstream decoder 461, andthe optical splitter 422 is omitted.

In Embodiment 3-2 depicted in FIG. 53, too, as is the case with the FIG.52 embodiment, the provision of total reflectors 451M and 451S at theoutput sides of the upstream mark encoder 441M and the upstream spaceencoder 441S permits removal of the optical input/output unit 420 andthe optical combiner 450, allowing reduction of the number of partsused. Moreover, it is also possible to form the switch 430 by an opticalsplitter and the optical combiner 450 by a switch.

FIG. 55 illustrates an example of the configuration of opticalcommunication equipment that is the communication partner of the opticalcommunication equipment shown in FIG. 53. The optical signals of theoptical frequencies f0 to fL from a light source 495 are switched by aswitch 435 between a downstream mark encoder 445M and a downstream spaceencoder 445S, depending on whether each piece of data of a downstreamdata from an input terminal 436 is mark or space. The filteringcharacteristic functions of the downstream mark encoder 445M and thedownstream space encoder 445S are chosen to be the same as the filteringcharacteristic functions IMi(f) and ISi(f) of the downstream markdecoder 461M and the downstream space decoder 461S of the partneroptical communication equipment. The downstream signal light from thedownstream mark encoder 445M and the downstream space encoder 445S isinput via an optical combiner 455 and an optical input/output unit 425to the optical fiber 410.

The upstream signal light input from the optical fiber 410 is input viathe optical input/output unit 425 and an optical splitter 426 to anupstream mark decoder 465M and an upstream space decoder 465S. Thefiltering characteristic functions of the decoders 465M and 465S arechosen to be the same as the filtering characteristic functions OMi(f)and OSi(f) of the upstream mark encoder 441M and the upstream spaceencoder 441S of the partner optical communication equipment. The outputsignal light from the upstream mark decoder 465M and the upstream spacedecoder 465S is input to optical detectors 475M and 475S, and outputelectrical signals from the optical detectors 475M and 475S are comparedby a comparator 485, which provides its compared output as an upstreamdata sequence to an output terminal 486.

According to Embodiment 3-2, even if multiple pairs of opticalcommunication equipment that outputs the downstream signal light and theoptical communication equipment that outputs the upstream signal lightshares the optical fibers 410, since each pair uses different codes(filtering functions), that is, the optical frequency characteristicfunctions bearing the afore-mentioned relationship, optical signals fromoptical communication equipment other than the pair of equipment alsoare orthogonal to each other and hence they do not become noise, andsince the downstream signal light and the upstream signal light are alsodifferent in code, at least one-half of the optical frequency componentsof the downstream signal lights can be modulated as upstream signallight. This effect is particularly effective in a PON (passive OpticalNetwork) configuration that is a point-to-N point network such asdefined in ITU-T Recommendation G. 983 and G. 984 series. In awavelength division multiplex-passive optical network (WDM-PON) of thetype heretofore proposed, downstream signal lights that are sent tooptical network units (ONU) which are other customer premises equipmentis usually mere noise, that is, noise that is merely discarded by anoptical filter or the like. In Embodiment 3-2, however, it can beeffectively used as lights for modulation as the upstream signal light.In the case of making an optical intensity design on the presumption ofmaking use of the downstream communication lights to other opticalnetwork unit, if the number of optical network units to be connected istoo small to secure downstream signal light of sufficient opticalintensity, it is necessary for the partner optical communicationequipment to output downstream light corresponding to the number ofunconnected optical network units.

Embodiment 3-3

Embodiment 3-3 uses trigonometric functions as the functions which havethe relationship of Eq. (22) or Eq. (23), Eq. (24) or Eq. (25), Eq. (26)or Eq. (27), and Eq. (28) or Eq. (29). That is, the functions for use inEmbodiment 3-3 are trigonometric functions that bear integral-multiplerelations between their period of optical intensity variation in theoptical frequency region or are phased π/2 apart when the periods ofoptical intensity variation in the optical frequency region when theperiods are the same. That is, the optical frequency characteristicfunction Mi(f) of the mark signal light, for instance, is given by Eq.(30)Mi(f)=(1+cos 2πsf/(fL−f0)+rπ/2))/2  (30)And the optical frequency characteristic function Si(f) of the spacesignal light is 1−Mi(f), that is, given by Eq. (31).Si(f)=1−(1+cos(2πsf/(fL−f0)+rπ/2))  (31)where s is an integer in the range from 1 to a value NI/2 obtained bydividing the maximum NI (required number of codes) by 2, r is 0 or 1,and fL−f0=FSR. In FIG. 6, (a), (b) and (c) are trigonometric functionswhich corresponds to s=1, 2 and 3 of Mi(f), respectively, the dottedlines corresponding to r=0 and the solid lines corresponding to r=1.

The filter of such an optical frequency characteristic function caneasily be formed by the Mach-Zehnder interferometer shown in FIG. 7, forinstance. FIG. 56 illustrates an example of the optical communicationequipment in such a case. The downstream signal light from the opticalfiber 410 is input via the optical input/output unit 420 to thedownstream decoder 461 and the upstream encoder 441 after being split bythe optical splitter 421. The downstream decoder 461 and the upstreamencoder 441 are each formed by the Mach-Zehnder interferometer. Theoptical path length differences between the optical paths 41 and 42 ofthe Mach-Zehnder interferometers forming the decoder 461 and the encoder441 are determined corresponding to the functions IMi(f) and OMi(f),respectively. Assuming that mark signal light is output from the oneoutput port of the coupler 44 of the decoder 461, space signal light isoutput from the other output port, and the optical outputs are input tothe optical detectors 470M and 470S, respectively. On the other hand,mark signal light from the one output port and space signal light fromthe other output port of the coupler 44 of the encoder 441 are providedto the switch 450. The illustrated optical communication equipment isidentical in construction and operation to the FIG. 53 equipment exceptthe above.

The use of such trigonometric functions also provides orthogonalitybetween respective codes, eliminates interference by other codes andexcludes the influence of reflected light, and in the point-to-N pointaccess network or the like the downstream signal lights from opticalcommunication equipment other than the partner one can also be used aslight for modulation as the upstream signal light; thus, this embodimentproduces the same effects as are obtainable with Embodiment 3-2.

Embodiment 3-4

Embodiment 3-4 is provided with an optical amplifier with a view toimplementing any one or all of: a solution to insufficient opticalintensity of the downstream signal light; reception of the downstreamsignal light with the optical intensity within the dynamic range of theoptical detector; and outputting of the upstream signal light with asufficient optical intensity.

As indicated by the broken line in FIG. 53, an optical amplifier 423 isinserted between the optical input/output unit 420 and the opticalsplitter 421 to amplify the downstream signal light. The opticalamplifier 423 amplifies input light from both directions; for example,SOA (Semiconductor Optical Amplifier) can be used. In this instance, theoptical amplifier 423 may be inserted in the optical fiber 410 asindicated by the broken line to amplify both of the downstream signallight and the upstream signal light. Alternatively, optical amplifiers423 a and 423 b may be connected to the two split output sides of theoptical splitter 421, respectively, and the amplification factor of theoptical amplifier 423 a is chosen so that the downstream signal lightfor reception use has an optical intensity in the dynamic range of eachof the optical detectors 470M and 470S, whereas the amplification factorof the optical amplifier 423 b for amplifying the downstream signallight for transmission use so that the upstream signal light has asufficient level of optical intensity. The optical amplifier 423 b maybe connected to the output side of the optical combiner 450. In thisinstance, the amplifier amplifies only light encoded by the encoder 441Mand 441S, and hence effectively amplifies them. Similarly, when opticalamplifiers 423 aM and 423 bS are connected to the output sides of thedownstream mark decoder 461M and the downstream space decoder 461S inplace of the optical amplifier 423 a, only decoded signal light can beamplified efficiently, and the optical amplifier 423 aM for mark and theoptical amplifier 423 aS for space can also be used as independent hardlimiters.

The optical combiner 450 may be configured as shown in FIG. 57, in whichthe optical signal outputs from the upstream mark encoder 441M and theupstream space encoder 441S are combined by an optical combiner 453after being amplified by optical amplifiers 452M and 452S, respectively,and the amplification factors of the optical amplifiers 452M and 452Sare controlled according to the data from the terminal 431 toselectively output coded light corresponding to mark and space, byincreasing the amplification factor of the optical amplifier 452M butdecreasing the amplification factor of the optical amplifier 452S whenthe data from the input terminal 431 is mark and by decreasing theamplification factor of the optical amplifier 452M but increasing theamplification factor of the optical amplifier 452S when the data isspace. In this way, the optical intensity of the upstream signal lightcan be increased sufficiently. In this case, an optical splitter can beused instead of the switch 430. This produces the effect of eliminatingthe need for using switches in the optical communication equipment.While in the above an embodiment of the optical communication equipmentof FIG. 53 has been described as using the optical amplifier, thefiltering characteristic functions of the encoder and the decoder foruse in the optical communication equipment may be not only the opticalfrequency chip sequence functions but also the trigonometric functionsdescribed previously with reference Embodiment 3-3. Furthermore, opticalamplifiers can similarly be used to effectively work even if they areinserted at respective places in the optical communication equipment inwhich total reflectors are provided at the output sides of the encoders,the optical input/output unit 420 is omitted and the downstream signallight is decoded as shown in FIG. 52. FIG. 58 illustrates an example ofsuch optical communication equipment, in which insertable opticalamplifiers are indicated by the broken lines and identified by the samereference numerals as those in FIGS. 52, 53 and 57, and no descriptionwill be given of them. In this instance, the optical splitter connecteddirectly to the optical input/output port 412 is formed by an opticalcombiner/splitter 421, and in the case of selecting the mark signallight and the space signal light by controlling, according to data, theamplification factors of the optical amplifiers 451M and 451S connectedto the sides opposite to the total reflectors 451M and 451S with respectto the mark encoder 441M and the space encoder 441S, the switch 430 isformed by an optical combiner/splitter. In the case where the markencoder 441M and the space encoder 441S are formed by asingle-structured encoder 441, the optical amplifiers 451M and 451S areinserted between the mark signal light output port and the space signallight output port of the encoder 441 and the total reflectors 451M and451S, respectively, and in the case of controlling these opticalamplifiers 451M and 451S according to the upstream data sequence, theinput port of the encoder 441 is connected directly to the opticalcombiner/splitter 421, and consequently the optical combiner/splitter430 can be omitted. In this case, the optical amplifiers 451M and 451Smay be substituted with switches which are ON/OFF-controlled inverselyto each other according to the upstream data; to sum up, they need onlyto select either one of the mark signal light and the space signal lightin accordance with the upstream data.

Embodiment 3-5

While in the above the receiving decoder circuit for the downstreamsignal light and the transmitting encoder circuit for the upstreamsignal light are provided in parallel, they may be provided in tandem.Referring now to FIG. 59, Embodiment 3-5 will be described below inwhich the transmitting encoder circuit is disposed near the opticalinput/output port 412, that is, on the side of the optical fiber 410,and the receiving decoder circuit is connected in cascade to thetransmitting encoder circuit.

To the optical input/output port 412 is connected the opticalcombiner/splitter 430 via an optical amplifier 442 as required.Accordingly, the downstream signal light from the optical fiber 410 isinput via the optical combiner/splitter 430 to the encoders 441M and441S. As referred to previously, the mark signal light and the spacesignal light are complementary to each other in the optical intensity ofeach optical frequency and equal in the mean optical intensities in theoptical frequency range from f0 to FL, and the optical frequenciescorresponding to one-half the optical frequency components forming themark signal or space signal light in the downstream signal light areutilized to form the mark signal light or space signal light as theupstream signal light.

Accordingly, the half of the effective optical frequency components inthe downstream signal light passes through the encoders 441M and 441S,and these transmitted optical outputs are combined by the switch 450 andinput to an optical combiner/splitter 424, wherein it is split for inputto the total reflector 451 and the optical splitter 422. The downstreamsignal light input to the optical splitter 422 is split into two forinput to the decoders 461M and 461S. If the optical frequencycharacteristic of this downstream signal light is matched to thefiltering characteristic function of the decoder 461M or 46 aS, theoptical frequency components, which is at least one-half those in thedownstream signal light in the optical fiber 210 as described previouslycorrectly pass through the decoder, providing decoded data from thecomparator 480.

On the other hand, the light reflected off the total reflector 451 isinput via the optical combiner/splitter 424 to the optical combiner 450.Since the switch 450 is controlled by the upstream data sequence fromthe terminal 431, the signal light encoded when the downstream signallight having passed through the encoder 441M or 441S previously isreflected off the total reflector 451, then subjected to the sameencoding, and input to the optical combiner/splitter 430, from which itis input as the upstream signal light to the optical fiber 410. Thisupstream signal light is encoded twice by the encoder 441M or 441S andthis encoding is based on the same characteristic, but there is a fearof the final encoding of the upstream signal light being affected by thepreceding light encoded at the time of passage through the switch 450from the optical fiber 410 and reflected by the total reflector 451—thisincurs the possibility of the optical frequency characteristic of theupstream signal light being disturbed. This can be avoided by amplifyingthe upstream signal light for input to the optical fiber 410 by theoptical amplifier 442 until it is saturated. However, this is limitedspecifically to the case of using a chip sequence as the function of thesignal light. It is also possible to exchange the opticalcombiner/splitter 430 for a switch and the switch 450 for an opticalcombiner/splitter. Furthermore, the afore-mentioned variousmodifications may similarly be effected. The switch 450 may beconfigured as shown in FIG. 57, in which the upstream signal light forinput to the optical fiber 410 is saturation-amplified by the opticalamplifiers 452M and 452S as a substitute for the optical amplifier 442.Furthermore, the optical amplifier 442 may be disposed at the stagepreceding the total reflector 451 as indicated by the broken line inFIG. 59. In this instance, the optical amplifier and the total reflectorcan be formed as a unitary part by giving total reflection coating toone end of SOA.

Embodiment 3-6

Embodiment 3-6 omits the optical combiner/splitter 424 and the totalreflector 451 in Embodiment 3-5 as shown in FIG. 60, for instance. Thedownstream signal light having passed through the encoders 441M and 441Sis input via the switch 450 to a partial reflector 454, and one portionof the downstream signal light is reflected by the partial reflector 454and the remaining portion of the signal light passes through the partialreflector and enters the optical splitter 422. In this instance, too, itwill easily be understood that the downstream signal light can becorrectly decoded and that one portion of the downstream signal lightcan be used to generate the upstream signal light. As compared with thecase of FIG. 59, this embodiment dispenses with the opticalcombiner/splitter 424, producing the effect of eliminating the loss inthe optical combiner/splitter 424. As indicated by the broken line, theoptical amplifier 442 may be disposed between the partial reflector 454and the switch 450. In this case, by depositing, for example, one end ofSOA with a partial reflection coating, the optical amplifier and thepartial reflector can be formed in a one-piece structure. The switch 450may be configured as shown in FIG. 57, in which the optical amplifiers451M and 451S are used also as the optical amplifier 442. Moreover, theafore-mentioned various modifications can similarly be effected.

Embodiment 3-7

In this embodiment a transmitter circuit is connected in cascade to theoutput side of a receiver circuit. As the optical detectors 470M and470S in the receiver circuit, optical detectors 471M and 471S are usedwhich are formed by optical amplifiers capable of taking out electricalsignals proportional to the input optical intensity, as shown in FIG.61. For example, SOAs can be used as the optical detectors 471M and471S, and the electrical signal from the optical detectors 471M and471S, which are proportional to the optical intensities of therespective optical outputs from the mark decoder 461M and the spacedecoder 461S, are input to the comparator 480. On the other hand, theamplified optical signals from the optical detector 471M and 471S arecombined by an optical combiner/splitter 472, and the combined output isinput to the switch 430. The other configurations and operations are thesame as those in FIG. 53. In this instance, too, since the opticalfrequency characteristics of each mark signal light and space signallight are formed as described previously, the signal light having passedthrough the downstream decoder 461 contains optical frequency componentscorresponding to the half of those of the downstream signal light, andthe upstream mark signal light or upstream space signal light can begenerated by the upstream encoder 441. With this configuration, theoptical splitter 421 for separating the downstream signal light to thereceiving side and the transmitting side in the configuration of FIG. 53becomes unnecessary, and the downstream signal light for input to theencoders 441M and 441S can be amplified by the optical detectors 471Mand 471S.

FIG. 62 illustrates an embodiment which uses the optical amplifiers 471Mand 471S as optical detectors in the case of using Mach-Zehnderinterferometers as the downstream decoder 461 and the upstream encoder441 as depicted in FIG. 56. The downstream signal light from the opticalinput/output unit 420 is input directly to the downstream decoder 461,and the decoded downstream mark signal and downstream space signal fromthe downstream decoder 461 are input to the optical detectors 471M and471S formed by optical amplifiers, respectively. The electrical signalsproportional to the optical intensities of the inputs to the opticaldetectors 471M and 471S are input to the comparator 480, and the marksignal light and the space signal light amplified by the opticaldetectors 471M and 471S are input to a correcting combiner 473 formed bya Mach-Zehnder interferometer having its port exchanged with that of theMach-Zehnder interferometer forming the downstream decoder 461. Thedifference in the optical path length between the optical paths 41 and42 in the downstream decoder 461 is corrected by the difference in theoptical path length between the optical paths 41 and 42 in thecorrecting combiner 471, and the downstream mark signal light and thedownstream space signal light are combined by the coupler 44 afterpassing through optical paths of the same length. The combineddownstream signal light is input to the encoder 441 formed by theMach-Zehnder interferometer. The other configurations and operations arethe same as shown in FIG. 56. Since the optical outputs from the opticaldetectors 471M and 471S are combined by the Mach-Zehnder interferometer,the loss can be reduced as compared with that in the case of using theoptical multiplexer 47 in FIG. 61.

Given below is a general description of the third mode of working of theinvention. This mode of working is predicated on the opticalcommunications system which: transmits the downstream signal light fromthe optical transmitter; receives the downstream signal light byreflective optical communication equipment; regenerates the downstreamdata sequence through utilization of part of the received downstreamsignal light; and modulates part of the received downstream signal lightinto the upstream signal light and transmits it to the above-mentionedoptical transmitter.

In this third mode of working of the invention, functions and theircomplementary functions need not always to be periodic. Accordingly, theoptical intensity-frequency characteristic of the received optical codesignal has the function Ci(f) or Ck(f) and the filtering frequencycharacteristic, the upstream encoder 441 is Cj(f) or Cm(f), and thesefunctions satisfy the following equations that express thescalar-product integration values for the interval of the opticalfrequency width FSR in an arbitrary range from the optical frequency Fstto Fla.∫Ci(f)·Cj(f)df=∫Ci(f)·Cm(f)df∫Ck(f)·Ci(f)df=∫Ck(f)·Cm(f)df

But it is not always required that Ck(f)=(1−Ci(f)) and Cm(f)=(1−Cj(f)).In the case were Ck(f)≠(1−Ci(f)), however, (1−Ci(f)) and (1−Ck(f)) arenot used in the same system. Similarly, when Cm(f)≠(1−Cj(f)), (1−Cj(f))and (1−Cm(f)) are not used in the same system.

Furthermore, at least one of the scalar-product integration values∫Ci(f)·Cj(f)df and ∫Ck(f)·Ci(f)df is not zero. That is, either Ci(f)>0or Ck(f)>0 holds.

When the functions Ci(f) and Cj(f) are periodic, it becomes as follows.

Letting a common multiple of the repletion period FSRi of the functionof each code in the optical frequency range from the optical frequencyFst to Fla be represented by the optical frequency width FSR and a valueobtained by dividing the optical frequency width FSR by a commonmultiple of the function repetition period FSRi be represented by theoptical frequency width FSRi,Ci(f)=Ci(f+FSRi), and∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Cj(f))df;for the j-th optical frequency characteristic function Cj(f) other thanthe i-th one,∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df.Based on the optical frequency characteristic functions bearing suchrelationships, for each piece of data of the binary data sequence theoptical frequency characteristic function of the received downstreamsignal light is made Ci(f) or (1−Ci(f)), depending on whether said pieceof data is mark or space. One portion of this downstream signal light isallowed to pass through the decoders whose filtering characteristicfunctions are Ci(f) and (1−Ci(f)), and the optical intensities of thetransmitted optical outputs are detected, and the received downstreamsignal light is regenerated as the mark or space in accordance with thedifference between the detected optical intensities.

And, for each piece of data of the upstream data sequence, the opticalfrequency characteristic of one portion of the downstream signal lightis modulated by an encoder to Cj(f) or (1−Cj(f)), or (1−Cj(f)) or Cj(f),depending on whether said each piece of data is mark or space, and thedownstream signal light is transmitted as an upstream signal light.

1. An optical communications system using optical codes, characterizedby: an optical transmitter which: emits from a light source an opticalsignal having an optical frequency width FSR contained in an opticalfrequency range from a predetermined optical frequency Fst to apredetermined optical frequency Fla; and provides said optical signal toan encoder formed by at least one of filter means whose opticalfiltering characteristic is a function Ci(f) or its complementaryfunction (1−Ci(f)) both corresponding to an i-th code at least in saidoptical frequency range from the predetermined optical frequency Fst tothe predetermined optical frequency Fla; and generates by and transmitsfrom said encoder an optical code signal whose opticalintensity-frequency characteristic is at least one of said functionCi(f) and its complementary function (1−Ci(f)) of said i-th codecorresponding to the value of each piece of data of said i-th binarydata sequence, at least in the optical frequency width FSR constained insaid optical frequency range from the predetermined optical frequencyFst to the predetermined optical frequency Fla; wherein: said functionCi(f) is a periodic function with an optical frequency f as a variable,expressed as Ci(f)=Ci(f+PFRi); the optical frequency width FSR is anoptical frequency width which is a common multiple of a repetitionperiod PFRi of a function forming each code in an optical frequencyrange from a predetermined optical frequency Fst to a predeterminedoptical frequency Fla; the complementary function of said function Ci(f)is a function obtained by subtracting the function Ci(f) from 1; saidfunction Ci(f) and said complementary function (1−Ci(f)) bear thefollowing relation:∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df where ∫df is a definite integral withrespect to f for an arbitrary interval corresponding to said opticalfrequency width FSR constained in said optical frequency range from theoptical frequency Fst to the optical frequency Fla; and the functionCi(f), a function Cj(f) of an arbitrary j-th code other than the i-thcode and the complementary function (1−Cj(f)) of said function Cj(f)bear the following relation:∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df; and an optical receiver whichincludes: at least optical filter means and an intensity detector fordetecting the optical intensity of a received optical signal; and which:generates from said received optical signal a first difference signalcorresponding to the difference between a first intensity signalcorresponding to the optical intensity of an optical signal whoseoptical intensity-frequency characteristic is Ci(f) based on saidfunction Ci(f) and a second intensity signal corresponding to theoptical intensity of an optical signal whose optical intensity-frequencycharacteristic is (1−Ci(f)) based on the complementary function(1−Ci(f)); and regenerates said data sequence from said first differencesignal.
 2. The optical communications system of claim 1, characterizedin that: said repetition period PFRi is an optical frequency widthobtained by dividing said optical frequency width FSR by an integer Nicorresponding to said function Ci(f); and let Δf represent the remainderof the division of an arbitrary optical frequency width equal to ornarrower than said optical frequency width FSR by the repetition periodPFRi of said function Ci(f), let 2π(Δf/PFRi) represent a phasedifference from said function Ci(f), and let Ci′(f)(=Ci(f+Δf)) representa function with an optical frequency (f+Δf) different by said remainderΔf from the optical frequency of said function Ci(f) of the i-th code;said functions Ci′(f), Cj(f) and (1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; said encoder is formed by filtermeans whose optical filtering frequency characteristic is said functionCi′(f) corresponding to each value of said remainder Δf transmittable insaid optical frequency range from the predetermined optical frequencyFst to the predetermined optical frequency Fla; said optical transmitteris a device which transmits as the optical code signal, an opticalsignal whose optical intensity-frequency characteristic is the functionCi′(f) of each value of said remainder Δf corresponding to the value ofeach piece of a binary data sequence, at least in the optical frequencywidth FSR; and said optical receiver is a device which regenerates saidbinary data sequence from said first difference signal corresponding toeach value of the remainder Δf which corresponds to the differencebetween: said first intensity signal generated from said receivedoptical signal and corresponding to the optical intensity of saidoptical signal whose optical intensity-frequency characteristic isCi′(f) based on each function Ci′(f) which corresponds to each value ofthe remainder Δf transmittable from said by the optical transmitter; andsaid second intensity signal generated from the received optical signaland corresponding to the optical intensity of the optical signal whoseoptical intensity-frequency characteristic is (1−Ci′(f)) based on thecomplementary function (1−Ci′(f)) corresponding to said function Ci′(f)which corresponds to each value of the remainder Δf.
 3. The opticalcommunications system of claim 1, characterized in that: said repetitionperiod PFRi is an optical frequency width obtained by dividing saidoptical frequency width FSFR by an integer Ni corresponding to saidfunction Ci(f); and let Δf represent the remainder of the division of anoptical frequency width equal to or narrower than said optical frequencywidth FSR by said period PFRi, let 2π(Δf/PFRi) represent a phasedifference from the function Ci(f), let said phase difference be set atπ/2, and let Ci′(f)(=Ci(f+Δf)) represent a function with an opticalfrequency (f+Δf) different by said remainder Δf from the opticalfrequency of said function Ci(f) of said i-th code; said functionsCi′(f), Cj(f) and (1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj′(f))df; said encoder is composed of: atleast one of filter means whose optical filtering frequencycharacteristic is said function Ci(f) or its complementary function(1−Ci(f)) both corresponding to said i-th code in said optical frequencyrange from the predetermined optical frequency Fst to the predeterminedoptical frequency Fla; and additional filter means whose opticalfiltering characteristic is either at least one of said functions Ci′(f)and (1−Ci′(f)), or at least one of said functions Cj(f) and (1−Cj(f));said optical transmitter is a device which: separates said binary datasequence by sequence converting means into a first separate datasequence and a second separate data sequence; generates and outputs, asthe optical code signal from said encoder, an optical signal obtained bycombining: a first optical signal whose optical intensity-frequencycharacteristic is either said function Ci(f) or (1−Ci(f)) set by saidfirst separate data sequence; and a second optical signal whose opticalintensity-frequency characteristic is either at least one of saidfunctions Ci′(f) and (1−Ci′(f)), or at least one of said functions Cj(f)and (1−Cj(f)) which correspond to the value of each piece of data ofsaid second separate data sequence; and said optical receiver is adevice which: detects from said received optical signal a seconddifference signal corresponding to the difference between a thirdintensity signal corresponding to the optical intensity of said opticalsignal whose optical intensity-frequency characteristic is Ci′(f) orCj(f) based on said function Ci′(f) or Cj(f) and a fourth intensitysignal corresponding to the optical intensity of said optical signalwhose optical intensity-frequency characteristic is (1−Ci′(f)) or(1−Cj(f)) based on the function Ci′(f) or Cj(f), respectively; andregenerates said first separate data sequence and said second separatedata sequence from said second difference signal and said firstdifference signal.
 4. The optical communications system of claim 3,characterized in that: said optical transmitter has means for separatingthe input binary data sequence by said sequence converting means into athird separate data sequence and a fourth separate data sequence, inaddition to said first and second separate data sequences, and includes:amplitude changing means by which said first optical signal, which hasits optical intensity-frequency characteristic set to be the functionCi(f) or (1−Ci(f)) according to the value of each piece of data of saidfirst separate data sequence, and said second optical signal, which hasits optical intensity-frequency characteristic set to be the functionCi′(f) or (1−Ci′(f)), or the function Cj(f) or (1−Cj(f)) according tothe value of each piece of data of said second separated data sequence,are controlled to have optical intensities corresponding to the value ofeach piece of data of said third separate sequence and the value of eachpiece of data of said fourth separate data sequence, respectively; andsaid optical receiver is a device which converts said first differencesignal and said second difference signal into digital values,respectively, and regenerates said first, second, third and fourthseparate data sequences from said digital values, respectively.
 5. Theoptical communications system of claim 1, characterized in that: saidoptical transmitter is a device which: receives an optical code signalwhose optical frequency width is at least FSR and whose opticalintensity-frequency characteristic is Cj(f) or (1−Cj(f)); and multipliessaid received optical code signal by at least one of said opticalintensity-frequency characteristics Ci(f), (1−Ci(f)), and zero inaccordance with the value of each piece of data of said binary datasequence, and outputs the multiplied received optical code signal. 6.The optical communications system of any one of claims 1 to 5,characterized in that: said period PFRi is an optical frequency widthobtained by dividing said optical frequency width FSR by an integer Nicorresponding to said function Ci(f); said functions Ci(f) and Cj(f)have either the periods PFRi and PFRj different from each other, orperiods PFRi and PFRj equal to each other, in which case let Δfrepresent the remainder of the division of an optical frequency widthequal to or narrower than said optical frequency width FSR by therepetition period PFRi of said function Ci(f), said function Cj(f) isCi′whose phase difference 2π(Δf/PFRi) from the function Ci(f) is π/2andsaid function Ci(f) is a function containing a sine or cosine function.7. The optical communications system of any one of claims 1 to 5,characterized in that: said optical frequency width FSR is divided intochips by a value V=2Si·Qi obtained by multiplying arbitrary integers Siand Qi both corresponding to said function Ci(f) by an integer 2; andsaid function Ci(f) is a function that repeats Qi times makingconsecutive Si chips have an optical intensity 1 and the succeeding Sichips have an optical intensity 0, or sequentially shifts the opticalfrequency positions of said consecutive Si chips of the opticalintensity 1 by a predetermined value.
 8. The optical communicationssystem of any one of claims 1 to 5, characterized in that: said opticalcommunications system is a two-way communication system; an opticaltransmitter of at least one side of said system is a device whichgenerates an optical code signal by making an optical signal have anoptical intensity-frequency characteristic by an encoder formed by atleast one encoding optical filter whose optical filtering frequencycharacteristics are said optical filtering frequency function Ci(f) andits complementary function (1−Ci(f)); and an optical receiver is adevice which separates optical code signals whose opticalintensity-frequency characteristics are Ci′(f) and (1−Ci(f)) from areceived optical signal by two decoding optical filters whose opticalfiltering characteristics are Ci′(f) and (1−Ci′(f)), or Cj(f) and(1−Cj(f)), where Ci′(f) is a function displaced a quarter period apartfrom Ci(f); said at least one encoding optical filter and said twodecoding optical filter are integrated on a monolithic planar lightwavecircuit substrate; and said optical communications system is providedwith: intensity detecting means for detecting the optical intensity of atransmitted optical signal from said at least one encoding opticalfilter or said two decoding optical filters; and controlling means forcontrolling the temperature of said monolithic planar lightwave circuitsubstrate to maximize the optical intensity to be detected.
 9. Theoptical communications system of any one of claims 1 to 5, characterizedin that: said optical receiver is a device which: divides said receivedoptical signal by filter means for each optical chip forming the code ofthe optical code signal; detects, as a chip intensity signal, theoptical intensity of each divided optical chip by an intensity detector;and delays, by delay means, such detected chip intensity signals of saidreceived optical signal corresponding to optical chips different in thetime of arrival from transmission lines so that said optical chipsarrive at the same time; and obtains the first difference signal bysubtracting, by means of an intensity difference detector, the summationof those of said delayed chip intensity signals whose function (1−Ci(f))corresponds to 1 from the summation of those of said delayed chipintensity signals whose function Ci(f) corresponds to
 1. 10. An opticaltransmitter, which: emits from a light source an optical signal havingan optical frequency width FSR contained in an optical frequency rangefrom a predetermined optical frequency Fst to a predetermined opticalfrequency Fla; and provides said optical signal to an encoder formed byat least one of filter means whose optical filtering characteristic is afunction Ci(f) or its complementary function (1−Ci(f)) bothcorresponding to an i-th code at least in said optical frequency rangefrom the predetermined optical frequency Fst to the predeterminedoptical frequency Fla; and generates by and transmits from said encoderan optical code signal whose optical intensity-frequency characteristicis at least one of said functions Ci(f) and (1−Ci(f)) of said i-th codecorresponding to the value of each piece of data of said i-th binarydata sequence, at least in said optical frequency width FSR contained insaid optical frequency range from the predetermined optical frequencyFst to the predetermined optical frequency Fla; wherein: said functionCi(f) is a periodic function with an optical frequency f as a variable,expressed as Ci(f)=Ci(f+PFRi); the optical frequency width FSR is theoptical frequency width which is a common multiple of a repetitionperiod PFRi of a function forming each code in said optical frequencyrange from the predetermined optical frequency Fst to the predeterminedoptical frequency Fla; the complementary function (1−Ci(f) of thefunction Ci(f) is a function obtained by subtracting said function Ci(f)from 1; said functions Ci(f) and (1−Ci(f)) bear the following relation:∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df where ∫df is a definite integral withrespect to f for an arbitrary interval corresponding to said opticalfrequency width FSR contained in the optical frequency range from theoptical frequency Fst to the optical frequency Fla; said function Ci(f),a function Cj(f) of an arbitrary j-th code other than the i-th code anda complementary function (1−Cj(f)) of said function Cj(f) bear thefollowing relation:∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df.
 11. The optical transmitter of claim10, characterized by the provision of: N encoders for generating andoutputting optical code signals whose optical intensity-frequencycharacteristics are different functions, respectively, said N being aninteger equal to or greater than 2; and a combiner for combining andtransmitting N sets of optical code signals.
 12. The optical transmitterof claim 10 or 11, characterized in that: letting a represent an integervalue from 1 to a value N/2 by dividing the code number N by an integer2, and letting r represent the remainder of division of 2, said functionCi(f) is as follows:(1+cos(2·π·a·f/FSR+r·π/2))/2.
 13. The optical transmitter of claim 10 or11, characterized in that: said optical frequency width FSR is dividedby an arbitrary integer R into chips; and said functions Ci(f) and Cj(f)are composed of “1” and “−1” chips.
 14. The optical transmitter of claim10 or 11, characterized in that: each encoder is provided with: a firstmodulation part for generating a first optical code signal whose opticalintensity-frequency characteristic is a code function assigned to saidencoder; a second modulation part for generating a second optical codesignal whose optical intensity-frequency characteristic is thecomplementary function of the function of said first modulation part;and a switch which outputs therethrough at least one of said first andsecond optical code signals by use of one of two values for each pieceof data of input binary data and outputs at least the other of saidfirst and second optical code signals by use of the other of said twovalues for each piece of data of said input binary data.
 15. The opticaltransmitter of claim 10, characterized in that: said repetition periodPFRi is an optical frequency obtained by dividing said optical frequencywidth FSR by an integer Ni corresponding to said function Ci(f); and letΔf represent the remainder of the division of an arbitrary opticalfrequency width equal to or narrower than said optical frequency widthFSR by the repetition period PFRi of said function Ci(f), let2π(Δf/PFRi) represent a phase difference from said function Ci(f), andlet Ci′(f)(=Ci(f+Δf)) represent a function with an optical frequency(f+Δf) different by said remainder Δf from the optical frequency of saidfunction Ci(f) of said i-th code; said functions Ci′(f), Cj(f) and(1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; said encoder is formed by filtermeans whose optical filtering frequency characteristic is said functionCi′(f) corresponding to each value of said remainder Δf transmittable atleast in the range from the predetermined optical frequency Fst to thepredetermined optical frequency Fla; said optical transmitter is adevice which transmits, for each piece of data of said binary datasequence, an optical signal whose optical intensity-frequencycharacteristic is said function Ci′(f) of the value of said remainder Δfcorresponding to the value of each piece of data, as said optical codesignal at least in said optical frequency width FSR; and said opticaltransmitter includes: a code modulation part which generates, for eachpiece of data of said binary data sequence, an optical code signal whoseoptical intensity-frequency characteristic is that one of functionssatisfying the conditions for the above-said relation which differs onlyin the phase Δf in accordance with the value of each piece of data; anda combiner for combining the optical code signals from said codemodulation parts and for outputting them as said output optical codesignal.
 16. The optical transmitter of claim 10, characterized in that:said period PFRi is an optical frequency width obtained by dividing saidoptical frequency width FSR by an integer Ni corresponding to saidfuntion Ci(f); let Δf represent the remainder of the division of anarbitrary optical frequency width equal to or narrower than said opticalfrequency width FSR by the repetition period PFRi of said functionCi(f), let 2π(Δf/PFRi) represent a phase difference from the functionCi(f), and let Ci′(f)(=Ci(f+Δf)) represent a function with an opticalfrequency (f+Δf) different by the remainder Δf from the opticalfrequency of said function Ci(f) of said i-th code; and said functionsCi′(f), Cj(f) and (1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; let the phase difference be set atπ/2; said encoder is composed of: at least one of filter means whoseoptical filtering frequency characteristic is said functions Ci(f) or(1−Ci(f)) both corresponding to said i-th code in said optical frequencyrange from the predetermined optical frequency Fst to the predeterminedoptical frequency Fla; and additional filter means whose opticalfiltering characteristic is either at least one of said functions Ci′(f)and (1−Ci′(f)), or at least one of said functions Cj(f) and (1−Cj(f));and said optical transmitter is provided with: a sequence convertingpart for separating said input binary data sequence into a firstseparate data sequence and a second separate data sequence; a firstmodulation part for generating a first optical signal whose opticalintensity-frequency characteristic is said function Ci(f) or (1−Ci(f)),depending on the value of each piece of data of said first separate datasequence; a second modulation part for generating a second opticalsignal whose optical intensity-frequency characteristic is at least oneof said functions Ci′(f) and (1−Ci′(f)), or at least one of saidfunctions Cj(f) and (1−Cj(f)), depending on the value of each piece ofdata of said second separate data sequence; a combiner for combiningsaid first and the second optical signals for outputting them as opticalcode signal.
 17. The optical transmitter of claim 16, characterized inthat: said sequence converting part is a converting part which convertsthe input binary data sequence into first, second, third and fourthseparate data sequences; said optical transmitter is provided with thirdand fourth modulation parts for modulating said first and second opticalsignals into signals of optical intensities corresponding to the valuesof respective pieces of data of said third and fourth separate datasequences, respectively; and said combiner combines said first andsecond optical signals of the light intensities corresponding to saidvalues, respectively.
 18. The optical transmitter of any one of claims15 to 17, characterized in that: said period PFRi is an opticalfrequency width obtained by dividing said optical frequency width FSR byan integer Ni corresponding to said function Ci(f); the periods of saidfunctions Ci(f) and Cj(f) are the periods PFRi and PFRj different fromeach other, or periods PFRi and PFRj equal to each other, in which caselet Δf represent the remainder of the division of an optical frequencywidth equal to or narrower than said optical frequency width FSR by therepetition period PFRi of said function Ci(f), said function Cj(f) isCi′(f) whose phase difference 2π(Δf/FSRi) from said function Ci(f) isπ/2, and said function Ci(f) is a function containing a sine or cosinefunction.
 19. The optical transmitter of any one of claims 15 to 17,characterized in that: said optical frequency width FSR is divided intochips by a value V=2Si·Qi obtained by multiplying arbitrary integers Siand Qi both corresponding to the function Ci(f) by an integer 2; andsaid function Ci(f) is a function that repeats Qi times makingconsecutive Si chips have an optical intensity 1 and the succeeding Sichips have an optical intensity 0, or sequentially shifts the opticalfrequency positions of said consecutive Si chips of the opticalintensity 1 by a predetermined value.
 20. The optical transmitter of anyone of claims 15 to 17, characterized in that there are provided foreach data sequence: said light source; an optical splitter for splittingan output optical signal from said light source into multiple opticalsignals; optical filters of optical filtering characteristics havingdifferent code functions, for receiving said split optical signals; anoptical combiner for combining said optical signals transmitted throughsaid optical filters and transmitting said combined output as an opticalcode signal; and code modulating means which is inserted between saidmultiple optical filters and said optical splitter or optical combinerand controlled by one of said multiple separate data sequences,respectively.
 21. An optical receiver characterized by: filter meanswhich permits the passage therethrough of an optical signal having anoptical intensity-frequency characteristic based on a function at leastin an optical frequency range from a predetermined optical frequency Fstto a predetermined optical frequency Fla; intensity detecting means fordetecting the optical intensity of said optical signal; and means foradding together or subtracting the intensity signals from each other;and which is supplied with the received optical signal and regeneratesdata corresponding to the difference between: a first intensity signalcorresponding to the optical intensity of an optical signal having anoptical intensity-frequency characteristic Ci(f) based on a Ci(f); and asecond intensity signal corresponding to the optical intensity of anoptical signal having an optical intensity-frequency characteristic(1−Ci(f)) based on the complementary function (1−Ci(f)) of said functionCi(f); wherein: said function Ci(f) is a periodic function expressed asCi(f)=Ci(f+PFRi), the value of the function Ci(f) being in the range of0 to 1; an optical frequency width FSR is an optical frequency widthwhich is a common multiple of a repetition period PFRi of a functionforming each code in said optical frequency range from the predeterminedoptical frequency Fst to the predetermined optical frequency Fla; saidcomplementary function of the function Ci(f) is a function obtained bysubtracting said function Ci(f) from 1; said functions Ci(f) and(1−Ci(f)) bear the following relation:∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df where ∫df is a definite integral withrespect to f for an arbitrary interval corresponding to said opticalfrequency width FSR contained in said optical frequency range from theoptical frequency Fst to the optical frequency Fla; and said functionCi(f), a function Cj(f) of an arbitrary j-th code other than said i-thcode and the complementary function (1−Cj(f)) of said function Cj(f)bear the following relation:∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df.
 22. The optical receiver of claim 21,wherein: said received optical signal is multiple optical code signalsencoded to have optical intensity-frequency characteristics that satisfyorthogonality relations; and said optical transmitter further comprisingmultiple decoders, each provided with: a splitter which is supplied withand splits said received optical signal into multiple optical signals; afirst filter which is supplied with one of said received optical signalssplit by said splitter and whose optical filtering characteristic isCi(f); a first intensity detector which is supplied with the output fromsaid first filter and detects its optical intensity as a first intensitysignal; a second filter which is supplied with one of said receivedoptical signal and whose optical filtering characteristic is (1−Ci(f));a second intensity detector which is supplied with the output from saidsecond filter and detects its optical intensity as a second intensitysignal; and an intensity difference detector which is supplied with saidfirst and second intensity signals and regenerates binary data based onthe intensity difference obtained by subtracting the one from the otherintensity signal, respectively; wherein said functions Ci(f) and(1−Ci(f)) differ between said multiple decoders.
 23. The opticalreceiver of claim 21 or 22, characterized in that: letting a representan integer value in the range from 1 to N/2 obtained by dividing thecode number N by an integer 2 and letting r represent the remainder ofthe division of 2, the function Ci(f) is as follows:(1+cos(2·π·a·f/FSR+r·π/2))/2.
 24. The optical receiver of claim 21 or22, wherein: said optical frequency width FSR is divided by an arbitraryinteger R into chips; and said function Ci(f) and the function Cj(f) arecomposed of “1” and “−1” chips; said optical receiver furthercomprising: a filter which is supplied with said received optical signaland divides and outputs said received input signal for each chip;multiple chip intensity detectors each of which is supplied with theoutput from said filter for each chip and detects the chip intensitysignal corresponding to the optical intensity of said optical signal foreach chip; and an intensity difference detector which is supplied withthe chip intensity signals from said multiple chip intensity detectors,and outputs binary data based on the summation of all the input chipintensity signals with that signal corresponding to each “1” chip ofsaid function Ci(f) held positive and that signal corresponding to each“1” chip of said function (1−Ci(f))held negative.
 25. The optical signalreceiver of claim 21, wherein: said PFRi is an optical frequency widthobtained by dividing said optical frequency width FSR by an integer Nicorresponding to said function Ci(f); and let Δf represent the remainderof the division of an optical frequency width equal to or narrower thansaid optical frequency width FSR by said repetition period PFRi, let2π(Δf/PFRi) represent a phase difference from the function Ci(f), andlet Ci′(f)(=Ci(f+Δf)) represent a function with an optical frequency(f+Δf) different by said remainder Δf from the optical frequency of saidfunction Ci(f) od said i-th code; said functions Ci′(f), Cj(f) and(1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; said optical signal receiverfurther comprising: a first filter which is supplied with said receivedoptical signal and whose optical filtering frequency characteristic issaid function Ci′(f) corresponding to each value of said remainder Δftransmittable from an optical transmitter of the communicating partner;a second filter which is supplied with said received optical signal andwhose optical filtering frequency characteristic is said function(1−Ci′(f)) corresponding to the function Ci′(f) which corresponds tosaid each value of said remainder Δf; a first intensity detectors whichare supplied with the output from said first filters and detect a firstintensity signals corresponding to the optical intensities of the outputfrom said first filters; a second intensity detectors which are suppliedwith the output from said second filters and detect a second intensitysignals corresponding to the optical intensities of the output from saidsecond filters; and means which are supplied with said first and secondintensity signals, detect the difference therebetween, and regeneratesand outputs the binary data sequence.
 26. The optical receiver of claim21, wherein: said PFRi is an optical frequency width obtained bydividing said optical frequency width FSR by an integer Ni correspondingto said function Ci(f); and let Δf represent the remainder of thedivision of an optical frequency width equal to or narrower than saidoptical frequency width FSR by the repetition period PFRi of saidfunction Ci(f), let 2π(Δf/PFRi) represent a phase difference from saidfunction Ci(f), let Ci′(f)(=Ci(f=Δf) represent a function with anoptical frequency (f=Δf) different by said remainder Δf from the opticalfrequency of said function Ci(f) of said i-th code, and let said phasedifference be set at π/2; said functions Ci′(f), Cj(f) and (1−Cj(f))bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; said optical receiver furthercomprising: a first filter which is supplied with said received opticalsignal and whose optical filtering frequency characteristic is saidfunction Ci(f); a second filter which is supplied with said receivedoptical signal and whose optical filtering frequency characteristic issaid function (1−Ci(f)); a first intensity detector which is suppliedwith the output from said first filter and detects a first intensitysignal corresponding to the optical intensity of the output from saidfirst filter; a second intensity detectors which are supplied with theoutput from said second filter and detects a second intensity signalcorresponding to the optical intensity of the output from said secondfilter; a third filter which is supplied with the received opticalsignal and whose optical filtering frequency characteristic is saidfunction Ci′(f) or Cj(f); a fourth filter which is supplied with saidreceived optical signal and whose optical filtering frequencycharacteristic is said function (1−Ci′(f)) or (1−Cj(f)); a thirdintensity detector which is supplied with the output from said thirdfilter and detects a third intensity signal corresponding to the opticalintensity of the output from said third filter; a fourth intensitydetector which is supplied with the output from said fourth filter anddetects a fourth intensity signal corresponding to the optical intensityof the output from said fourth filter; a first subtractor which issupplied with said first and second intensity signals and outputs thedifference therebetween as a first difference signal; a secondsubtractor which is supplied with said third and fourth intensitysignals and outputs the difference therebetween as a second differencesignal; and data generating means which is supplied with said first andsecond difference signals and outputs said binary data sequence.
 27. Theoptical receiver of claim 26, characterized in that: said datagenerating means is means which renders said first difference signalinto first binary data and said second difference signal into secondbinary data, and arranges said first binary data and said second binarydata in a sequential order to form said binary data sequence.
 28. Theoptical receiver of claim 26, characterized in that: said datagenerating means is provided with: a first A/D converter for convertingsaid first difference signal to a first digital; a second A/D converterfor converting said second difference signal to a second digital; andbinary sequencing means which is supplied with said first and seconddigital signals, and outputs that one of predetermined combinations offour or more pieces of data 0 or 1 for a combination of the values ofsaid input digital signals.
 29. The optical receiver of any one ofclaims 25 to 28, characterized in that: said functions Ci(f) and Cj(f)have either the periods PFRi and PFRj different from each other, orperiods PFRi and PFRj equal to each other, in which case let Δfrepresent the remainder of the division of an optical frequency widthequal to or narrower than said optical frequency width FSR by therepetition period PFRi of said function Ci(f), and said function Cj(f)is Ci′(f)(=Ci(f ±π/2) whose phase difference 2π(Δf/PFRi) from saidfunction Ci(f) is π/2.
 30. The optical receiver of any one of claims 25to 28, wherein: said optical frequency width FSR is divided into chipsby a value V=2Si·Qi obtained by multiplying arbitrary integers Si and Qiboth corresponding to said function Ci(f) by an integer 2; and saidfunctions Ci(f) and Cj(f) are composed of “1” and “−1” chips; saidoptical receiver further comprising: a filter which is supplied withsaid received optical signal and divides and outputs said received inputsignal for each chip; multiple chip intensity detectors each of which issupplied with the filter output for each chip and detects the chipintensity signal corresponding to the optical intensity of said opticalsignal for each chip; and an intensity difference detector which issupplied with the chip intensity signals from said multiple chipintensity detectors, and outputs binary data based on the summation ofall the input chip intensity signals with that signal corresponding toeach “1” chip of said function Ci(f) held positive and that signalcorresponding to each “1” chip of said function (1−Ci(f)) held negative.31. Reflective optical communication equipment which is supplied with areceived optical signal and a binary data sequence, modulates thereceived optical signal to an optical signal whose opticalintensity-frequency characteristic is a function with an opticalfrequency f as a variable, and transmits the modulated optical signal,and which is characterized by: an encoder which is supplied with saidreceived optical signal of at least an optical frequency width FSRcontained in an optical frequency range from a predetermined opticalfrequency Fst to a predetermined optical frequency Fla and outputs anoptical signal filtered by optical filter means whose optical filteringfrequency characteristic is a first function Ci(f) in said opticalfrequency range; a complementary encoder which is supplied with saidreceived optical signal and outputs a complementary optical signalfiltered by optical filter means whose optical frequency characteristicis the complementary function (1−Ci(f)) in said optical frequency range;and selective combining means which selectively combines, according tothe value of each piece of data, the output optical signal from saidencoder and the complementary optical signal from said complementaryencoder, and transmits them as an optical code signal; wherein: saidfunction Ci(f) is a periodic function expressed as Ci(f)=Ci(f+PFRi), thevalue of said function Ci(f) being in the range of 0 to 1; said opticalfrequency width FSR is an optical frequency width which is a commonmultiple of a repetition period PFRi of a function forming each code insaid optical frequency range from the predetermined optical frequencyFst to the predetermined optical frequency Fla; the complementaryfunction of said function Ci(f) is a function obtained by subtractingsaid function Ci(f) from 1; said functions Ci(f) and (1−Ci(f)) bear thefollowing relation:∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df where ∫df is a definite integral withrespect to f for an arbitrary interval corresponding to said opticalfrequency width FSR contained in said optical frequency range from theoptical frequency Fst to the optical frequency Fla; and said functionCi(f), a function Cj(f) of an arbitrary j-th code other than said i-thcode and the complementary function (1−Cj(f)) of said function Cj(f)bear the following relation:∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df.
 32. The reflective opticalcommunication equipment of claim 31, which is characterized by: adecoder which is supplied with said received optical signal and whoseoptical filtering frequency characteristic is said function Cj(f)); acomplementary decoder which is supplied with said received opticalsignal and whose optical filtering frequency characteristic is saidfunction (1−Cj(f)); a first optical detector which is supplied with theoutput light from said decoder and outputs an intensity signalcorresponding to the optical intensity of the output light from saiddecoder; a complementary optical detector which is supplied with theoutput light from said complementary decoder and outputs a complementaryintensity signal corresponding to the optical intensity of the outputlight from said complementary decoder; and a comparator which issupplied with said intensity signal and said complementary intensitysignal and outputs one of two pieces of data in accordance with thelevel difference between said intensity signals when the differenceexceeds a predetermined value.
 33. The reflective optical communicationequipment of claim 32, characterized in that said selective combiningmeans is provided with: a total reflector and a complementary totalreflector for totally reflecting said received optical signal,respectively; and selectors and complementary selectors disposed betweensaid encoder and and said total reflector and between said complementaryencoder and said complementary total reflector, respectively, forselecting either one of said optical signal and an optical signalcomplementary thereto in accordance with the value of input data. 34.The reflective optical transmission equipment of claim 33, which ischaracterized by: optical amplifiers for use as said optical detectorand said complementary optical detector for optically amplifying theinput optical signals thereto and outputting the amplified opticalsignals and intensity signals corresponding to the optical intensitiesof said input optical signals; and an optical combiner for combining theamplified optical signals from said optical detector and saidcomplementary optical detector and inputting the combiner optical signalas said received optical signal to said encoder and said complementaryencoder.
 35. The reflective optical communication equipment of claim 33,which is characterized by: a switch for selecting said optical signal orcomplementary optical signal in accordance with the value of input data;an optical combiner/splitter which is supplied with the output from saidswitch, splits said output into two, and inputs one of them to saiddecoder and said complementary decoder; and a total reflector which issupplied with the other split light from said optical combiner/splitterand totally reflects the input light.
 36. The reflective opticalcommunication equipment of claim 32, characterized by: a switch forselecting said optical signal or said complementary optical signal inaccordance with the value of input data; and a partial reflector whichis supplied with the output light from said switch, reflects a portionof the output light and inputs the remaining portion of said outputlight to said decoder and said complementary decoder.
 37. The reflectiveoptical communication equipment of any one of claims 32 to 36,characterized in that: said period PFRi is an optical frequency widthobtained by dividing said optical frequency width FSR by an integer Qicorresponding to said function Ci(f); let Δf represent the remainder ofthe division of an optical frequency width equal to or narrower thansaid optical frequency width FSR by the repetition period PFRi of thefunction Ci(f); said functions Ci(f) and Cj(f) have either the periodsPFRi and PFRj different from each other or periods PFRi and PFRj equalto each other; said function Cj(f) is Ci′(f) whose phase difference2π(Δf/PFRi) from said function Ci(f) is π/2; and said function Ci(f) isa trigonometric function; said encoder and said complementary encoderare integrated as an output encoder; and said decoder and saidcomplementary decoder are integrated as an input decoder.
 38. Thereflective optical communication equipment of any one of claims 32 to36, characterized in that: said optical frequency width FSR is dividedinto chips by a value V=2Si·Qi obtained by multiplying arbitraryintegers Si and Qi both corresponding to said function Ci(f) by aninteger 2; and said function Ci(f) is a function that repeats Qi timesmaking consecutive Si chips have an optical intensity 1 and thesucceeding Si chips have an optical intensity 0, or sequentially shiftsthe optical frequency positions of said consecutive Si chips of theoptical intensity 1 by a predetermined value; said encoder and saidcomplementary encoder are integrated as an output encoder; and saiddecoder and said complementary decoder are integrated as an inputdecoder.
 39. An optical communications system using optical codes,characterized by: an optical transmitter provided with: multiple lightsources for emitting optical signals of optical frequenciescorresponding to MU =V chips each having a chip width that is a unitoptical frequency width into which an optical frequency width FSRcontained in an optical frequency range from a predetermined opticalfrequency Fst to a predetermined optical frequency Fla is divided by anatural number M and an integer U equal to or greater than 3; drivesignal generators for generating drive signals for said multiple lightsources; an optical combiner for combining the output lights from saidmultiple light sources and outputting the combined light as an opticalcode signal; and code modulating means which is inserted between saidmultiple light sources and said drive signal generators or said opticalcombiner and controlled by each piece of data of an i-th binary datasequence to make said optical code signal have an opticalintensity-frequency characteristic based on at least one of said i-thcode function Ci(f) and its complementary function (1−Ci(f)); wherein:said optical frequency width FSR is an optical frequency width which isa common multiple of a repetition period PFRi of a function forming eachcode in said optical frequency range from the predetermined opticalfrequency Fst to the predetermined optical frequency Fla; saidcomplementary function of the function Ci(f) is a function obtained bysubtracting said function Ci(f) from 1; said functions Ci(f) and(1−Ci(f)) bear the following relation:∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df where ∫df is a definite integral withrespect to f for an arbitrary interval corresponding to said opticalfrequency width FSR contained in said optical frequency range from theoptical frequency Fst to the optical frequency Fla; and said functionCi(f), a function Cj(f) of an arbitrary j-th code other than said i-thcode and the complementary function (1−Ci(f)) of said function Cj(f)bear the following relation:∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df; and an optical receiver whichincludes: at least optical filter means and an intensity detector fordetecting the optical intensity of the optical signal received by saidoptical receiver; and which: generates from said received optical signala first difference signal corresponding to the difference between afirst intensity signal corresponding to the optical intensity of anoptical signal whose optical intensity-frequency characteristic is Ci(f)and a second intensity signal corresponding to the optical intensity ofan optical signal whose optical intensity-frequency characteristic is(1−Ci(f)); and regenerates said data sequence from said first differencesignal.
 40. The optical communications system of claim 39, characterizedin that: the chip number Pi of the period PFRi of said function Ci(f) isthe number of chips forming the optical frequency width obtained bydividing the chip number V of said optical frequency width FSR by aninteger Ni corresponding to said function Ci(f); let Δf represent theremainder of the division of an arbitrary chip number equal to orsmaller than V by said chip number Pi of the repetition period PFRi ofsaid function Ci(f), let 2π(Δf/Pi) represent a phase difference fromsaid function Ci(f), and let Ci′(f) (=Ci(f=Δ)) represent a function withan optical frequency different by said remainder Δf from the opticalfrequency of said function Ci(f) of said i-th code; said functionsCi′(f), Cj(f) and (1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; the control by said code modulatingmeans is to make the optical intensity-frequency characteristic of saidoptical code signal have the function Ci′(f) corresponding to each valueof said remainder Δf that can be transmitted; said optical transmitteris a device which transmits as said optical code signal, an opticalsignal whose optical intensity-frequency characteristic is the functionCi′(f) of the value of said remainder Δf corresponding to the value ofeach piece of binary data sequence, at least in the optical frequencywidth FSR; and said optical receiver is a device which regenerates saiddata sequence from each first difference signal corresponding to eachvalue of said remainder Δf which corresponds to the difference between:said first intensity signal generated from said received optical signaland corresponding to the optical intensity of the optical signal whoseoptical intensity-frequency characteristic is Ci′(f) based on eachfunction Ci′(f) which corresponds to each value of said remainder Δftransmittable by said optical transmitter; and said second intensitysignal generated from said received optical signal and corresponding tothe optical intensity of the optical signal whose opticalintensity-frequency characteristic is (1−Ci(f)) based on thecomplementary function (1−Ci′(f)) corresponding to said function Ci′(f)which corresponds to the value of said remainder Δf.
 41. The opticalcommunications system of claim 39, characterized in that: the chipnumber Pi of the period PFRi of said function Ci(f) is the number ofchips forming the optical frequency width obtained by dividing the chipnumber V of said optical frequency width FSR by an integer Nicorresponding to said function Ci(f); let Δf represent the remainder ofthe division of a chip number equal to or smaller than V by said chipnumber Pi of the repetition period PFRi of said function Ci(f), let2π(Δf/Pi) represent a phase difference from said function Ci(f), let thephase difference be set at π/2, and let Ci′(f) (=Ci(f+Δf)) represent afunction with an optical frequency (f+Δf) different by said remainder Δffrom the optical frequency of said function Ci(f) of said i-th code;said functions Ci′(f), Cj(f) and (1−Ci(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; said code modulating means is meanswhich selectively controls each chip to have the 1 or 0 level inaccordance with a function; said code modulating means is means whichgenerates a first optical signal whose optical intensity-frequencycharacteristic is at least one of said function Ci(f) and itscomplementary function (1−Ci(f)) both corresponding to said i-th codeand a second optical signal whose optical intensity-frequencycharacteristic is at least one of said function Ci′(f) and saidcomplementary function (1−Ci′(f)), or at least one of said functionsCj(f) and (1−Cj′(f)); said optical transmitter is a device which:separates said binary data sequence by sequence converting means into afirst separate data sequence and a second separate data sequence;combines the first optical signal whose optical intensity-frequencycharacteristic is said function Ci(f) or (1−Ci(f)) set by said firstseparate data sequence and the second optical signal whose opticalintensity-frequency characteristic is at least one of said functionsCi′(f) and (1−Ci′(f)), or at least one of said functions Cj(f) and(1−Cj(f)) corresponding to the value of each piece of data of saidsecond separate data sequence; and outputs the combined optical signalas said optical code signal; and said optical receiver is a devicewhich: detects from its received optical signal, a second differencesignal corresponding to the difference between a third intensity signalcorresponding to the optical intensity of the optical signal whoseoptical intensity-frequency characteristic is Ci′(f) or Cj(f) based onsaid function Ci′(f) or Cj(f) and a fourth intensity signalcorresponding to the optical signal whose optical intensity-frequencycharacteristic is (1−Ci′(f)) or (1−Cj(f)) based on said complementaryfunction (1−Ci′(f)) or (1−Cj(f)), respectively; and regenerates saidfirst separate data sequence and said second separate data sequence fromsaid first difference signal and said second difference signal,respectively.
 42. The optical communications system of claim 41,characterized in that: said optical transmitter has means for separatingthe input binary data sequence by said sequence converting means into athird separate data sequence and a fourth data sequence, in addition tosaid first and second separate data sequences, and includes: amplitudechanging means by which said first optical signal which has its opticalintensity-frequency characteristic set to be the function Ci(f) or(1−Ci(f) according to the value of each piece of data of said firstseparate data sequence and said second optical signal which has itsoptical intensity-frequency characteristic set to be the function Ci′(f)or (1−Ci′(f)), or the function Cj(f) or (1−Cj(f)) according to the valueof each piece of data of said second separate data sequence, arecontrolled to have optical intensities corresponding to the value ofeach piece of data of said third separate data sequence and the value ofeach piece of data of said fourth separate data sequence, respectively;and said optical receiver is a device which converts said first andsecond difference signals into digital values, respectively, andregenerates said first, second, third and fourth separate data sequencesfrom said digital values, respectively.
 43. The optical communicationssystem of any one of claims 39 to 42, characterized in that: the chipnumber V for dividing said optical frequency width FSR is a value 2Si·Qiobtained by multiplying arbitrary integers Si and Qi both correspondingto said function Ci(f) by an integer 2; and said function Ci(f) is afunction that repeats Qi times making consecutive Si chips have anoptical intensity 1 and the succeeding Si chips have an opticalintensity 0, or sequentially shifts the optical frequency positions ofsaid consecutive Si chips of optical intensity 1 by a predeterminedvalue.
 44. The optical communications system of any one claims 39 to 42,characterized in that: said optical receiver is a device which: dividessaid received optical signal by filter means for each optical chipforming the code of the optical code signal; detects, as a chipintensity signal, the optical intensity of each divided optical chip byan intensity detector; and delays, by delay means, such detected chipintensity signals of said received optical signal corresponding tooptical chips different in the time of arrival from transmission linesso that said optical chips arrive at the same time; and obtains thefirst difference signal by subtracting, by means of an intensitydifference detector, the summation of those of said delayed chipintensity signals whose function (1−Ci(f)) corresponds to 1 from thesummation of those of said delayed chip intensity signals whose functionCi(f) corresponds to
 1. 45. An optical transmitter, comprising: multiplelight sources for emitting optical signals of optical frequenciescorresponding to MU =V chips each having a chip width that is a unitoptical frequency width into which an optical frequency width FSRcontained in an optical frequency range from a predetermined opticalfrequency Fst to a predetermined optical frequency Fla is divided by anatural number M and an integer U equal to or greater than 3; drivesignal generators for generating drive signals for said multiple lightsources; an optical combiner for combining the output light from saidmultiple light sources and outputting it as an optical code signal; andcode modulating means which is inserted between said multiple lightsources and said drive signal generators or said optical combiner andcontrolled by each piece of data of an i-th binary data sequence to makesaid optical code signal have an optical intensity-frequencycharacteristic based on at least one of said i-th code function Ci(f)and its complementary function (1−Ci(f)); wherein: said opticalfrequency width FSR is an optical frequency width which is a commonmultiple of a repetition period PFRI of a function forming each code insaid optical frequency range from the predetermined optical frequencyFst to the predetermined optical frequency Fla; the complementaryfunction of said function Ci(f) is a function obtained by subtractingsaid function Ci(f) from 1; said functions Ci(f) and (1−Ci(f)) bear thefollowing relation:∫Ci(f)·Ci(f)df>∫Ci(f)·(1−Ci(f))df where ∫df is a definite integral withrespect to f for an arbitrary interval corresponding to said opticalfrequency width FSR contained in said optical frequency range from theoptical frequency Fst to the optical frequency Fla; and said functionCi(f), a function Cj(f) of an arbitrary j-th code other than said i-thcode and the complementary function (1−Cj(f)) of the said function Cj(f)bear the following relation:∫Ci(f)·Cj(f)df=∫Ci(f)·(1−Cj(f))df.
 46. The optical transmitter of claim45, characterized by the provision of: N sets of light sources and drivesignal generators for generating and outputting optical code signalshaving optical intensity-frequency characteristics of differentfunctions, respectively, said N being an integer equal to or greaterthan 2; and a combiner for combining and transmitting N sets of opticalcode signals.
 47. The optical transmitter of claim 45 or 46,characterized in that: said functions Ci(f) and Cj(f) are each composedof “1” and “1”chips.
 48. The optical transmitter of claim 45 or 46,characterized in that: each code modulating means is provided with: afirst modulation part for generating a first optical code signal whoseoptical intensity-frequency characteristic is a code function assignedto said encoder; a second modulation part for generating a secondoptical code signal whose optical intensity-frequency characteristic isthe complementary function of said function of said first modulationpart; and a switch which outputs therethrough at least one of said firstand second optical code signals by use of one of two values for eachpiece of data of input binary data and outputs at least the other ofsaid first and second optical code signals by use of the other of saidtwo values for each piece of data of said input binary data.
 49. Theoptical transmitter of claim 45, characterized in that: the chip numberPi of the period PFRi of said function Ci(f) is the number of chipsforming the optical frequency width obtained by dividing the chip numberV of said optical frequency width FSR by an integer Ni corresponding tosaid function Ci(f); let Δf represent the remainder of the division ofan arbitrary chip number equal to or smaller than V by the chip numberPi of the repetition period PFRi of said function Ci(f), let 2π(Δf/Pi)represent a phase difference from said function Ci(f), and let Ci′(f)(=Ci(f+Δf)) represent a function with an optical frequency (f+Δ)different by said remainder Δf from the optical frequency of saidfunction Ci(f) of said i-th code; said functions Ci′(f), Cj(f) and(1−Cj(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; the control by said code modulatingmeans is to make the optical intensity-frequency characteristic of saidoptical code signal have said function Ci′(f) corresponding to eachvalue of said remainder Δf that can be transmitted; said opticaltransmitter is a device which transmits, for each piece of data of saidbinary data sequence, as said optical code signal, an optical signalwhose optical intensity-frequency characteristic is said function Ci′(f)of the value of said remainder Δf corresponding to the value of eachpiece of data, at least in said optical frequency width FSR, and whichincludes: a code modulation part which generates, for each piece of dataof said binary data sequence, an optical code signal whose opticalintensity-frequency characteristic is that one of functions satisfyingthe conditions for the above-said relation which differs only in thephase Δf in accordance with the value of each piece of data; and acombiner for combining the optical code signals from said codemodulation parts and outputting the combined signal as the outputoptical code signal.
 50. The optical transmitter of claim 45,characterized in that: the chip number Pi of the period PFRi of saidfunction Ci(f) is the number of chips forming the optical frequencywidth obtained by dividing the chip number V of said optical frequencywidth FSR by an integer Ni corresponding to the function Ci(f); let Δfrepresent the remainder of the division of a chip number equal to orsmaller than V by the chip number Pi of the repetition period PFRi ofsaid function Ci(f), let 2π(Δf/Pi) represent a phase difference fromsaid function Ci(f), and let Ci′(f) (=Ci′(f+Δf) represent a functionwith an optical frequency (f+Δ) different by said remainder Δf from theoptical frequency of said function Ci(f) of said i-th code; saidfunctions Ci′(f), Cj(f) and (1−Ci(f)) bear the following relation:∫Ci′(f)·Cj(f)df=∫Ci′(f)·(1−Cj(f))df; let the phase difference be set atπ/2; said code modulating means is means which generates a first opticalsignal whose optical frequency characteristic is said function Ci(f) or(1−Ci(f)) both corresponding to said i-th code and a second opticalsignal whose optical frequency characteristic is at least one of saidfunctions Ci′(f) and (1−Ci′(f)), or at least one of said functions Cj(f)and (1−Cj(f)); and said optical transmitter is provided with: a sequenceconverting part which separates said input binary data sequence into afirst separate data sequence and a second separate data sequence; afirst modulation part which generates a first optical signal whoseoptical intensity-frequency characteristic is said function Ci(f) or(1−Ci(f)), depending on the value of each piece of data of said firstseparate data sequence; a second modulation part which generates asecond optical signal whose optical intensity-frequency characteristicis at least one of said functions Ci′(f) and (1−Ci′(f)), or at least oneof said functions Cj(f) and (1−Cj′(f)), depending on the value of eachpiece of data of said second separate data sequence; and a combiner forcombining said first and second optical signals and outputs the combinedsignal as the optical code signal.
 51. The optical transmitter of claim50, characterized in that: said sequence converting part is a convertingpart which converts said input binary data sequence into, first, second,third and fourth separate data sequences; said optical transmitter isprovided with third and fourth modulation parts for modulating saidfirst and second optical signals into signals of optical intensitiescorresponding to the values of respective pieces of data of said thirdand fourth separate data sequences; and said combiner is a combiner forcombining said first and second optical signals of the light intensitiescorresponding to said values, respectively.
 52. The optical transmitterof any one of claims 49 to 51, characterized in that the chip number Vfor dividing said optical frequency width FSR is a value 2Si·Qi obtainedby multiplying arbitrary integers Si and Qi both corresponding to saidfunction Ci(f) by an integer 2; and said function Ci(f) is a functionthat repeats Qi times making consecutive Si chips have an opticalintensity 1 and makes the succeeding Si chips have an optical intensity0, or sequentially shifts the optical frequency positions of saidconsecutive Si chips of the optical intensity 1 by a predeterminedvalue.